WO1991012199A1

WO1991012199A1 - Electrical device for loading of hydrogen and its isotopes to high activities in hydrogen permeable media

Info

Publication number: WO1991012199A1
Application number: PCT/CA1991/000048
Authority: WO
Inventors: Michael J Dignam
Original assignee: Dignam, Michael, J.
Priority date: 1990-02-15
Filing date: 1991-02-13
Publication date: 1991-08-22
Also published as: AU7236091A

Abstract

An electrical device is described for loading and maintaining, in a hydrogen permeable material, hydrogen activities orders of magnitude higher than those achievable by direct equilibration. A hydrogen permeable storage electrode, electronically coupled to an external power supply is surrounded by an electronically insulating and hydrogen ion permeable spacer material which in turn is surrounded by an electronically conducting source electrode which is permeable to hydrogen and is electronically insulated by the spacer material from the storage electrode. Application of a positive potential to the source electrode with respect to the storage electrode results in hydrogen ions flowing from the source electrode to the storage electrode through the spacer material.

Description

TITLE: Electrical device for loading of hydrogen and its isotopes to high activities in^~hydrogen permeable media

FIELD OF THE INVENTION

This invention relates to an apparatus for establishing and maintaining high activities of hydrogen, its isotopes and mixtures thereof, henceforth referred to simply as hydrogen, in stable media permeable to hydrogen.

BACKGROUND OF THE INVENTION

There are three known methods for loading hydrogen into hydrogen permeable materials, usually metals. The first involves direct equilibration of said materials with hydrogen gas. The hydrogen permeable material is placed in direct contact with hydrogen atmosphere whereupon hydrogen enters that metal lattice as atomic hydrogen after dissociating into atoms on the metal surface. The equilibrium state is described by

a„/a_a ⁰= ( - )¹

where a_u is the hydrogen atom activity in the metal, a_g the hydrogen molecule activity in the gas and the superscript o denotes the reference state corresponding to one atmosphere pressure of molecular hydrogen. Thus a 10 fold increase in a_B requires a 100 fold increase in a_g or approximately a 100 fold increase in the hydrogen pressure if the pressure is not too high.

The second method is also a gas phase method but employs the technique of ion beam implantation where a beam of ionized hydrogen

BSTITUTE SHEET atoms at energies higher than thermal energies are directed at the hydrogen permeable metal.

The third method utilizes an electrochemical cell comprised of the metal to be loaded with hydrogen as one electrode (in the field of electrochemistry referred to as the cathode) biased negative with respect to a counter electrode (similarly referred to as the anode), with both being in spaced apart configuration and immersed in an aqueous mixed ionically conducting electrolyte. When a sufficient bias is applied, electrolysis takes place with oxygen gas being evolved at the anode while at the cathode hydrogen is simultaneously evolved as hydrogen gas and is injected into the cathode material as atomic hydrogen. The simultaneous evolution of oxygen and hydrogen gas amounts to the electrolysis of water. If the overpotential associated with the evolution gas is n_c, then thermodynamic analysis places an upper bound on the hydrogen atom activity in the metal cathode, a₀, given by &_a/a. < e^qπckτ where q is the charge on the proton, k is Boltzman's constant and T the absolute temperature. To maximize a_m by this electrochemical method requires a large n_c and hence conditions far from equilibrium which involve substantial cell currents and total cell over potentials substantially greater than n_a. In principle, a,,, can be maximized for a given current density by treating the cathode surface and choosing the electrolyte composition to maximize n_c.

The major shortcoming of the electrochemical approach is that the desired process is not the primary process; it is secondary to the process of electrolysis or electrolyte breakdown which is in direct competition and interferes with the hydrogen loading process. To achieve and maintain high and constant __., requires a condition which places the system far from equilibrium and therefore requires a substantial power input into the system. Dissipation of the bulk of this power in the system leads to degradation of various cell components over time, through heating and/or dissolution mechanisms. It is desirable to design a system in which the primary process is the loading of hydrogen into the medium of interest, in contrast to the electrochemical method.

SUMMARY OF THE INVENTION

The subject invention is an apparatus for loading hydrogen permeable materials with hydrogen, comprising a source electrode, an electronically insulating and hydrogen ion permeable electrode spacer, and a storage electrode. The two electrodes are electronically conducting and permeable to hydrogen. Electronically conducting refers to a material with a low electronic resistivity, there being a high concentration of electrons which conduct the electronic current. Electronically insulating refers to a material with a high electronic resistivity, being characterized by a low concentration of electrons which normally conduct the electronic current. The term hydrogen permeable, when applied to the source and storage electrodes means permeable to hydrogen atoms or ions but not necessarily permeable to molecular hydrogen. The electrode spacer is an electronic insulator which is permeable to hydrogen ions and effectively impermeable to atomic and molecular hydrogen. The two electrodes are separated by and in close contact with the hydrogen ion permeable electrode spacer with which they can exchange hydrogen ions. The hydrogen source is in close contact with and able to exchange hydrogen with the source electrode. The cell geometry is such as to provide no effective path for the transport of hydrogen out of the storage electrode except through the electrode spacer as hydrogen ions. This is preferably accomplished by enveloping the storage electrode by the spacer material except for an electrical contact area, and in turn enveloping the spacer coated storage electrode by the source electrode which contact the source medium on its exterior surface.

To load the storage electrode with hydrogen, a positive potential is applied to the outer source electrode with respect to the inner storage electrode. This potential drop gives rise to an electric field in the electronically insulating spacer material so that positively charged ions are driven from the source electrode to the storage electrode. Hydrogen is ionized at the source electrode into which it dissolves and through which it diffuses and the hydrogen ions at the source electrode-spacer interface, are driven by the electric field through the spacer material to the storage electrode. The electrons liberated by the hydrogen atoms at the source electrode flow into the storage electrode through the external circuit thereby maintaining charge neutrality in the storage electrode. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the following drawings, in which:

Figure 1 is a perspective view with a cutaway of a preferred embodiment of the hydrogen loading apparatus of the present invention.

Figure 2 illustrates a cross sectional side view of the same preferred embodiment of the hydrogen loading apparatus of the present invention.

Figure 3 illustrates a sectional view of the same preferred embodiment of a hydrogen loading apparatus of the present invention taken along the line A - A of Figure 2.

Figure 4 illustrates a cross sectional view of an alternative embodiment of the hydrogen loading apparatus of the present invention.

Figure 5 illustrates a cross sectional view of another alternative embodiment of the hydrogen loading apparatus of the present invention comprising a storage medium embedded entirely within the storage electrode.

Figure 6 illustrates a cross sectional view of another view embodiment of the hydrogen loading apparatus of the present

BSTITUTE SHEET 6 invention wherein the storage medium is enveloped by the storage electrode and the hydrogen impermeable electronic conductor.

Figure 7 illustrates a cross section view of another alternative embodiment of the hydrogen loading apparatus of the present invention wherein there are a plurality of separated storage media embedded within and enveloped by the storage electrode and the hydrogen impermeable electronic conductor.

Figure 8 illustrates a cross sectional view of another alternative embodiment of the hydrogen loading apparatus of the present invention comprising a hydrogen storage medium situated between a hydrogen impermeable barrier and the storage electrode.

Figure 9 illustrates a cross sectional view of an alternative embodiment of the hydrogen loading apparatus comprising an electrochemically controllable hydrogen source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 illustrates a preferred embodiment of a hydrogen loading apparatus of the subject invention. Hydrogen loading cell 10 comprises an electronically conductive hydrogen permeable storage electrode 12 conjoined at both ends by hydrogen impermeable electronic conductors 14a and 14b. In close physical contact with and completely enveloping the storage electrode 12 and an arbitrary length of the hydrogen impermeable conductors 14 is the 7 electronically insulating and hydrogen ion conducting spacer material 16. Enveloping the spacer material and in close physical contact with it is hydrogen permeable source electrode 18, being of substantially the same length as and aligned with storage electrode 12. A thin hydrogen dissociation catalyst 20 may or may not be present on the outer surface of source electrode 18.

Figure 2 shows a cross sectional side view of the hydrogen loading apparatus of the present invention from which it should be clear that the hydrogen impermeable conductors 14a, 14b serve two purposes, the first being to act as a barrier to hydrogen diffusing out the ends of storage electrode 12, a possibility which arises because storage electrode 12 must be electronically coupled to external power supply 22 during operation. From Figure 3 which shows the section view of the hydrogen loading apparatus shown in Figure 2 along the line A - A, it can be seen that in operation hydrogen ions are driven radially inwards to the storage electrode 12 through spacer material 16 from source electrode 18.

The operation of cell 10 will now be described in detail with respect to Figure 2. A convenient source medium is hydrogen gas. On shorting electrodes 12 and lit the system reaches an equilibrium state in which the chemical potential of hydrogen is the same throughout the cell. The hydrogen atom activity in storage electrode 12 is represented by a_Ha ^* for these conditions. On applying a positive potential, V_a, to source electrode 18 versus storage electrode 12, a potential gradient is established across electrode spacer material 16 producing an electric field in spacer

TE SHEET material 16, resulting in the flow of hydrogen ions from source electrode 18 to storage electrode 12. This is accompanied by a flow of electrons through the external circuit of power supply 22 from source electrode 18 to storage electrode 12 so that overall, hydrogen is transported from source electrode 18 to storage electrode 12. If the potential is held constant, cell 10 reaches an equilibrium state with respect to the hydrogen ion distribution in which the diffusional force driving the hydrogen ions from storage electrode 12 to source electrode 18 balances the applied electrical force driving the hydrogen ions in the opposite direction. Through standard thermodynamic argument, the hydrogen atom activity in the storage electrode 12 for these conditions is given by

qVa/KT

At room temperature, a^ increases by approximately a factor of 10 for every 60 mV increase in V_s. This expression is also valid for negative V_B which when applied following e.g. shorting of the cell electrodes 12 and 18 causes hydrogen to be pumped out of storage electrode 12 into source electrode 18 and into the source medium. For positive V_B, the upper limit achievable for a_Ha is determined by the ability of cell 10 to withstand mechanical rupture. For a cell constructed of ideal materials (spacer material 16 being a perfect electronic insulator and there being no diffusional leakage of hydrogen) the cell current would be zero under the conditions of equilibrium with respect to the hydrogen ion distribution. In practice, small leakage currents will always be found which have

EET two components, one due to the finite electronic conductivity of spacer material 16, the other with replacing the hydrogen lost through diffusional leakage.

The key features of a cell geometry which will result in high hydrogen activities in storage electrode 12 are:

1. The electrode spacer material 16 must be non porous, essentially electronically insulating, possess an adequate breakdown voltage and be permeable to hydrogen ions while essentially blocking to all other ions present.

2. All paths from storage electrode 12 to source electrode 18, source or the ambient must be effectively impermeable to hydrogen in atomic and molecular form.

3. The materials making up the cell must be sufficiently elastic or plastic to withstand the volume expansion that accompanies buildup of hydrogen activity within the storage electrode.

4. The cell 10 must have sufficient tensile strength to contain the pressure buildup within the storage electrode associated with the desired hydrogen activity.

5. The materials making up the storage and source electrodes 12, 18 must be sufficiently permeable to hydrogen.

TE SHEET There are many metals that are permeable to hydrogen which can be used to form the source and storage electrodes 12 and 18 including titanium, palladium, tantalum, tungsten, niobium, vanadium, and alloys and metal mixtures of the same in addition to alloys of silver with palladium, alloys of iron and titanium and so on. Molecular hydrogen from the source medium must be dissociated into hydrogen atoms at the outer surface of the source electrode. While some metals which are permeable to hydrogen atoms are also adequate hydrogen dissociation catalysts, in general this is not the case. To overcome this limitation on the choice of source electrode materials, a thin layer of a hydrogen dissociation catalyst, e.g. platinum, can be applied to the outer surface of source electrode 18. Most metals are sufficiently permeable to hydrogen to function as a source electrode 18 provided they are in thin film form and the cell 10 is operating at the appropriate temperature with hydrogen dissociation catalyst 20 present. The electronically conducting and hydrogen impermeable metal used to electronically couple storage electrode 12 to external power supply 22 is preferably a metal from the group of metals exhibiting low hydrogen permeability comprising nickel, copper and platinum.

Many metal oxides in thin film form are electronic insulators but permeable to hydrogen ions and thus can serve as electrode spacer material 16. Many of these oxides do not provide charge compensation for the hydrogen ions resulting in poor hydrogen ion conduction through thick specimens due to space charge buildup. This problem can be avoided by forming spacer material 16 as a thin film. Alternatively, a charge compensated hydrogen ion

T conductor, such as B- alumina, may serve as spacer material 16.

Next to molecular hydrogen, the most natural choice for the source medium is water in liquid or vapour form. Under these conditions for ideal operation the interface between the source and source electrode should catalyze the dissociation of water molecules into free molecular oxygen and hydrogen in source electrode and the reverse reaction, thus an oxygen evolution/reduction electrocartalyst is indicated. However, as long as the oxygen evolution process can proceed at an appreciable rate, the hydrogen atom activity in the storage electrode for V_β positive is given by

where a_Ha° is the activity of hydrogen atoms in the storage electrode when it is equilibrated directly with molecular hydrogen at 1 atmosphere pressure and the temperature in question and __ V_e is equal to or greater than the reversible potential for the electrolysis of water to produce molecular hydrogen at one atmosphere pressure and molecular oxygen at its partial pressure or activity in the source medium. The equality applies if the catalyst equilibrates the water dissociation reaction and its reverse under condition of essentially zero net reaction rate, the inequality otherwise. With suitable catalysts and/or high enough operating temperature, other substances can act as the hydrogen source, for example methane gas and other hydrocarbons, metal hydrides, ammonia, organic acids, alcohols and aldehydes and so on,

SUBSTITUTE SHEET where again any isotope of hydrogen may be present.

The operating temperature range of the cell is limited on the low temperature side by the need to maintain appreciable mobility of hydrogen or hydrogen ions throughout cell 10 and on the high temperature side by the thermal stability and structural properties of the materials chosen to form cell 10.

There are several possible ways to construct hydrogen loading cell 10. Two examples utilizing the same cell geometry but different spacer growth processes are discussed below but are not deemed limiting thereof.

1. Using a wire for a cylindrically shaped storage electrode 12, electronic conductors 14a and 14b of the same diameter and low hydrogen permeability, for example nickel, are conjoined to both ends of storage electrode 12, by e.g. plasma welding.

2. This composite element is then coated with metal oxide spacer material 16 by atomic reactive sputter disposition from a metal target. This is accomplished by suitably masking off the composite wire element so that storage electrode 12 plus an arbitrary length of the nickel end pieces 14a, 14b at each end of the storage electrode 12 are exposed. A uniform film is obtained by rotating this composite element at some angular velocity under the metal target. 3. To form source electrode 18, the spacer coated storage element is then appropriately masked off and the source electrode metal is deposited by atomic sputtering to the desired thickness, with the spacer coated storage element being uniformly rotated under the source metal target to give a uniform coating.

4. A thin coating of a hydrogen decomposition catalyst 20 may then be deposited onto this source electrode surface in the same way if so desired.

An alternative method to that described above involves growing metal oxide spacer material 16 by electrochemical anodic oxidation of metal, as follows.

1. Repeat step 1 above.

2. This metal wire is suitably masked off and the metal from which the metal oxide is to be formed is deposited by atomic sputtering to the desired thickness.

3. This coated metal wire is then suitably masked off and one end of the wire is electronically coupled to a power supply while the other end is immersed into an aqueous solution of appropriate composition and containing a suitable counter electrode. This element is then anodically biased whereupon the metal anodically oxidizes to produce metal oxide spacer material 16.

UBSTITUTE SHEET 4. The source metal electrode 18 is then deposited as in step 4 of the above procedure and similarly, the hydrogen decomposition catalyst may be deposited as in step 5.

More efficient means of storing hydrogen will become more important as the use of hydrogen increases as a source of energy. The present invention shows how any hydrogen permeable material may be loaded with hydrogen to activities in excess of those achievable by direct equilibration of the material with gaseous hydrogen. This invention could also be useful in applications where high hydrogen activities may be required to initiate fusion of hydrogen isotopes and mixtures thereof.

Figures 4 to 9 illustrate alternative embodiments of the hydrogen loading apparatus of the present invention. Figure 4 illustrates an alternative geometry cell 30 employing spherical storage electrode 32 joined to a hydrogen impermeable conductor 34 and enveloped in spacer material 36 which in turn is partially enveloped by source electrode 38 which may or may not be coated with catalyst 39.

Figure 5 illustrates another alternative embodiment of the hydrogen loading apparatus of the subject invention. Hydrogen loading cell 40 is similar in geometry to loading cell 10. However, cell 40 also comprises a storage medium 41 permeable to hydrogen embedded entirely within storage electrode 42. Storage medium 41 may or may not be an electronic conductor, the only requirements it must satisfy being that it must be permeable to hydrogen and stable enough to withstand high hydrogen activities. One possible medium other than a hydrogen permeable metal which can be employed as a storage medium is vacuum.

Figure 6 illustrates a further embodiment of the hydrogen loading apparatus of the subject invention. In hydrogen loading cell 50, hydrogen permeable storage medium 51 is not embedded entirely within storage electrode 52 but is surrounded by both storage electrode 52 and hydrogen impermeable electronic conductor 54a, 54b.

Figure 7 illustrates yet another alternative embodiment of the hydrogen loading apparatus of the subject invention. In hydrogen loading cell 60, a plurality of hydrogen permeable storage media 61 substantially within storage electrode 62 are shown, the rest of the hydrogen loading cell 60 features being identical to hydrogen loading cell 30.

Figure 8 illustrates still another alternative embodiment of the hydrogen loading apparatus of the subject invention. Hydrogen loading cell 70 comprises hydrogen storage medium 72 situated between hydrogen permeable storage electrode 74 and hydrogen impermeable barrier 76 which is adapted to permit electrical coupling between storage electrode 74 and external power supply 78. Spacer material 80 is situated between storage electrode 74 and source electrode 82 which is adapted to be electronically connected to power supply 78 and may or may not be coated with a hydrogen dissociation catalyst 84.

SUBSTITUTE SHEET Figure 9 illustrates an alternative embodiment of the subject hydrogen loading apparatus wherein cell 90 is adapted to be immersed in an aqueous electrolyte 93 wherein an appropriate potential is applied between source electrode 92 and a suitable counterelectrode 94 such that the source electrode potential V₉₂ is in the vicinity of the hydrogen reversible potential. Controlling the potential between source electrode 92 and counterelectrode 94 allows control over the hydrogen concentration in source electrode 92, which in turn permits control over the rate of hydrogen ion flow across spacer material 96 to storage electrode 98, a feature which is equivalent to controlling the hydrogen source pressure in the preferred embodiment. The potential between the source electrode 92 and storage electrode 98, V₉₂-V₉₈ is controlled independently of the potential between source electrode 92 and counterelectrode 94, V₉₄-Vg₂. The hydrogen impermeable electronic conductors 100a, 100b, and those exposed areas of the spacer material 96 not coated by source electrode 92 are coated with electrically insulating coatings 102a, 102b, suitably chosen to be chemically stable in the aqueous medium being utilized.

While the present invention has been described and illustrated with respect to the preferred and alternative embodiments, it should be understood that numerous variations of these embodiments may be made without departing from the scope of the invention, which is defined in the appended claims.

Claims

I CLAIM:

1. An apparatus for loading hydrogen, comprising:

(a) an electronically conductive source electrode permeable to hydrogen;

(b) an electronicallyconductive storage electrodemeans permeable to hydrogen spaced from the source electrode for receiving and storing hydrogen;

(c) an electronically insulating spacer means permeable to hydrogen ions and impermeable to atomic and molecular hydrogen situated between and in close contact with the source electrode and the storage electrode means for electronically insulating the storage electrode means from the source electrode and for providing a flow path for hydrogen ions to be driven from the source electrode to the storage electrode means when the source electrode is biased positive with respect to the storage electrode means;

(d) a hydrogen impermeable electronic conductor means contacting a portion of the storage electrode means not contacting the spacer means for electronic coupling to the storage electrode means;

(e) wherein the geometry and material properties of the apparatus are selected to provide no effective path for the transport of hydrogen out of the storage electrode means except as hydrogen ions through the spacer means.

2. The apparatus as defined in claim 1 wherein any portion of the storage electrode means not contacting the spacer means is contacted by the electronic conductor means.

3. The apparatus as defined in claim 1, wherein the storage electrode means comprises an electronically conducting storage electrode and a hydrogen storage means permeable to hydrogen situated substantially within the storage electrode.

4. The apparatus as defined in claim 3 wherein the storage means is in contact with the hydrogen impermeable electronic conductor and/or the spacer means.

5. Apparatus for loading hydrogen, comprising:

(a) an electronically conducting storage electrode permeable to hydrogen;

(b) a hydrogen impermeable electronic conductor joined to the electronically conducting storage electrode and adapted to be electrically coupled to a power supply; (c) an electronically insulating spacer material permeable to hydrogen ions enveloping the storage electrode and a portion of the electronic conductor adjacent to the connection to the storage electrode;

(d) an electronically conducting source electrode adapted to be electronically coupled to a power supply, permeable to hydrogen and enveloping all of the spacer material except that portion of the spacer material enveloping the electronic conductor whereby when the source electrode is biased with a positive potential with respect to the storage electrode, hydrogen ions are forced through the spacer material to the electronically conducting storage electrode.

6. The apparatus as defined in claim 1 wherein the storage electrode means is made from a hydrogen permeable metal, metal alloy or metal mixture formed from the class of metals containing vanadium, titanium, tantalum, palladium, tungsten and niobium.

7. The apparatus as defined in claim 5 wherein the electronically conductive storage electrode material is made from a hydrogen permeable alloy formed from the class of metals including titanium, iron, tungsten, palladium tantalum, titanium, vanadium, and silver.

SUBSTITUTE SHEET

8. The apparatus as defined in claim 5 wherein the storage electrode is a hydrogen permeable metal from the class of metals containing titanium, tantalum, vanadium, tungsten, palladium and niobium.

9. The apparatus as defined in claim 3 wherein the storage means is vacuum.

10. The apparatus according to claim 5 wherein the spacer material is an electronically insulating metal oxide or mixed metal oxide.

11. The apparatus according to claim 10 wherein the spacer material is an electronically insulating metal oxide or mixed metal oxide formed from the class of metal oxides containing tantalum oxide, niobium oxide, tungsten oxide, hafnium oxide, zirconium oxide, magnesium oxide and aluminum oxide.

12. The apparatus as defined in claim 5 wherein the source electrode is made from an electronically conducting and hydrogen permeable metal formed from the class of metals containing tantalum, palladium, vanadium, titanium, niobium and tungsten.

TITUTE SHEET

13. The apparatus as defined in claim 5 wherein the source electrode is made from an electronically conducting and hydrogen permeable metal alloy or metal mixture formed from the class of metal containing tantalum, palladium, vanadium, titanium, niobium, tungsten, silver and iron.

14. The apparatus as defined in claim 5 wherein the electronic conductor is made from a metal from the class of metals including platinum, copper and nickel.

15. Apparatus for loading hydrogen, comprising:

(a) an electronically conducting and hydrogen permeable cylindrical metal wire conjoined at both ends by an electronically conducting and hydrogen impermeable cylindrical metal wire;

(b) an electronically insulating spacer material permeable to hydrogen ions and impermeable to atomic and molecular hydrogen, enveloping the entire length of the hydrogen permeable metal wire and a portion of the hydrogen impermeable conductor conjoined at both ends of the hydrogen permeable metal wire; and

(c) an electronically conducting source electrode, permeable to hydrogen and enveloping the spacer material

SUBSTITUTE SHEET and storage electrode and being of essentially the same length as and aligned with the storage electrode, whereby when the source electrode is biased with a positive potential with respect to the storage electrode, hydrogen ions are driven radially inwards from the source electrode to the storage electrode.

16. The apparatus as defined in claim 15 wherein the electronically conductive storage electrode wire is palladium or tantalum.

17. The apparatus as defined in claim 15 wherein the spacer material is an oxide of tantalum, niobium, zirconium or aluminum.

18. The apparatus as defined in claim 15 wherein the source electrode is palladium or tantalum.

19. The apparatus as defined in claim 15 wherein the hydrogen impermeable electronic conductor is nickel, platinum or copper.

SUBSTITUTE SHEET