Thermally Actuated Hydrogen Secondary Battery

The present invention relates to a secondary (rechargeable) battery and more particularly to a thermally actuated or activated hydrogen secondary battery which operates on the principle of intercalation/deintercalation of hydrogen ions (H.sup.+) or protons into/from the active materials of the two electrodes.

A secondary battery of the lead-acid type suffers from a number of drawbacks. Thus, its open current voltage, mid-range voltage and energy density are relatively low. Moreover, such a battery is relatively heavy and large.

It is the principal object of the present invention to provide a secondary battery, which compared to a lead-acid secondary battery, has a relatively high open current voltage, mid-range voltage and energy density and is relatively lightweight and smaller. A further object is to provide means for converting solar thermal energy or waste heat directly into electrical energy via an electrochemical reaction.

The secondary battery of the present invention is a thermally actuated hydrogen secondary battery which comprises

(a) an anode having a thermally actuated anode-active solid reversible hydrogen-containing inorganic compound which deintercalates hydrogen ions (H.sup.+) in a discharge cycle when exposed to thermal energy,

(b) a compatible gas-free electrolyte capable of transporting hydrogen ions (H.sup.+) between the anode and a cathode when the anode is exposed to thermal energy, and

(c) the cathode having a hydrogen ion-intercalatable cathode-active material which intercalates hydrogen ions (H.sup.+) in a discharge cycle when the anode is exposed to thermal energy.

Suitable thermally actuated anode-active solid reversible hydrogen-containing inorganic compounds are exemplified below. "Reversible" compounds are compounds which can deintercalate or release hydrogen ions (H.sup.+) or protons when exposed to thermal energy in a discharge cycle of the battery leaving behind a host material which can intercalate or insert hydrogen ions (H.sup.+) or protons when the thermal energy is removed and/or when electrical work is applied to the battery in a charge cycle.

Suitable hydrogen ion-intercalatable cathode-active materials are exemplified below.

The gas-free electrolyte or proton ionic conductor is either a liquid or a solid. Suitable gas-free liquid or solid electrolytes are exemplified below.

The liquid electrolyte can be buffered. The solute in the above nonaqueous liquid electrolytes can bear n-butyl, isobutyl, sec-butyl or similar functional group substitutions. The electrolyte must be a "compatible electrolyte", i.e., it must not cause the decomposition of the anode-active or cathode-active materials. For example, an aqueous electrolyte would not be used with a NaH or a MgH.sub.2 anode-active material, because NaH (sodium hydride) and MgH.sub.2 (magnesium hydride) decompose in water.

The polymer complex solid electrolyte can be prepared by dissolution of CF.sub.3 SO.sub.3 H and the linear polymer in tetrahydrofuran as a mutual solvent (distilled from sodium benzophenone) and evaporation of the solvent to form a thin film using a mole ratio of about 0.25-5.1 of H.sup.+ cations of the CF.sub.3 SO.sub.3 H to one mole of the repeat unit of the phosphazene polymer and a mole ratio of about 0.10-5.1 of H.sup.+ cations of the CF.sub.3 SO.sub.3 H to one mole of the repeat unit of the polyalkylene oxide polymer. Repeat unit mole is equivalent to the molecular weight of the polymer repeat unit, e.g., --CH.sub.2 CH.sub.2 O--.sub.n using polyethylene oxide as an example or --N=P (OEt OEt OMe).sub.2 --.sub.n using poly(di[2(2-methoxyethoxy)ethoxy]phosphazene as an example. The polymer complex of poly(di[2-(2-methoxyethoxy)ethoxy]phosphazene) is used in cells operating at temperatures between about 10.degree. C.-100.degree. C., while the other polymer complexes can be substituted therefor in cells operating at temperatures between about 100.degree. C.-150.degree. C. These polymer complexes are proton ionic conductors due to the mobile hydrogen cation (H.sup.+) or proton and the mobile anion (CF.sub.3 SO.sub.3.sup.-) and hence they provide a path between the electrodes for the migration of deintercalated hydrogen ions or protons.

In a preferred embodiment of the invention, the anode and cathode have catalytic films thereon which improve charge transfer kinetics. Such catalytic films are made of an oxide, such as ruthenium dioxide (RuO.sub.2), rhenium dioxide (ReO.sub.2) or iridium dioxide (IrO.sub.2), and/or a metal, such as palladium (Pd) or platinum black (Pt).

In the anode and cathode of the secondary battery, the cathode-active material can be codispersed with a carbon black depolarizer using an EPDM (ethylene-propylene-diene monomer) or TFE (tetrafluoroethylene) binder with a final configuration of the electrode in a retriculated foam (80%-97% open porosity) or the anode-active and cathode-active materials can be applied to the surface of a foam or retriculated metal-like current collector, such as a platinum-plated nickel foam of 3% density. In operation of the secondary battery, both the anode and cathode are ohmically connected to the external circuit through the porous current collector by means that avoid rectifying diode behavior.

The secondary battery has an anode and a cathode, both preferably having catalytic films thereon, with the gas-free liquid or solid electrolyte or proton ionic conductor therebetween. The optional catalytic films, which can be formed by evaporation of tetrahydrofuran slurries thereof, can be bindered to the electrodes by the same polymer complex material used in making the solid electrolyte. The electrodes are separated by a cell separator to prevent ohmic contact between the electrodes. These components are enclosed within an envelope. The envelope at least adjacent the anode is made of a heat-absorbant material, such as pyrolytic grade graphite or reduced metal intercalate of graphite, so that the anode can be thermally actuated during discharge.

These secondary batteries are actuated in the discharge cycle by impinging thermal energy (heat) onto the anode or onto the total cell, such as thermal energy from waste heat or solar thermal irradiation. Laser energy or microwave energy can also be used. The thermal energy input is either ambient incident irradiation or concentrated above ambient energy. The cells having a liquid electrolyte or a solid electrolyte are actuated and discharge at cell operating temperatures between 40.degree. C.-50.degree. C. or between 10.degree. C.-150.degree. C., respectively. The cells are charged in the charge cycle by the removal of thermal energy (heat) and/or by the addition of electrical work. During discharging, hydrogen ions (H.sup.+) or protons are deintercalated or released from the anode-active material and intercalated (inserted or topotactically reacted) into the cathode-active material; during charging, the reverse reaction occurs, i.e., hydrogen ions (H.sup.+) or protons are deintercalated or released from the original cathode-active material and intercalated (inserted or topotactically reacted) into the original anode-active material.

The principles of operation of the thermally actuated hydrogen secondary battery are illustrated by the following half cell and overall cell discharge cycle reactions for a cell having a gas-free liquid or solid electrolyte or proton ionic conductor, a VNbZrH.sub.3 anode and a V.sub.6 O.sub.13 cathode. ##STR1##

The overall cell discharge cycle reaction for this cell is: ##STR2## With the removal of heat (-.DELTA.) and/or the application of electrical work, the following reverse half cell and overall cell charge cycle reactions occur: ##STR3##

Overall cell discharge cycle reactions and calculated energy density for various exemplary thermally actuated hydrogen secondary batteries of the invention are as follows:

With the removal of the heat (-.DELTA.) and/or the application of electrical work, the reverse charge cycle reactions occur.

The above calculation of the cell energy density is accomplished by the following relationship:

The MRV is normally defined from a cell discharge curve.

The advantages of the thermally actuated hydrogen secondary battery of the invention over the conventional lead-acid storge battery are tabulated below.

A thermally actuated hydrogen secondary battery designed and fabricated to deliver 12 volts can contain 30% the number of cells, typically weigh 5%-25% as much, occupy 10%-30% the volume and deliver twelve times the energy density of a lead-acid battery.

The thermally actuated hydrogen secondary battery of the invention can be used for converting solar thermal energy or waste heat into electrical energy, for utility load levelling, in electrical vehicles and in military applications. The device of the invention can also be used as an electrochemical sensor for the determination of hydrogen if a metallic electrode is substituted for the reversible hydrogen-containing inorganic compound or can function as an electrochemical sensor for the determination of temperature, with presently indicated cathodes, by the direct relationship between temperature and hydrogen ion (H.sup.+) activity as reflected by the change in potential of the cell in the 30.degree. C. to 250.degree. C. temperature range. As a sensor the cell can be miniaturized and placed on an integrated circuit chip and used as a smart sensor.