BACKGROUND OF THE INVENTION

1. Fiel d of the Invention

[0001] The invention relates generally to a device for storage and conversion of energy, and more particularly to a device capable of absorbing electric, electromagnet ic or caloric energy, storing the absorbed energy, and releasing the absorbed energy, for example as electric or as chemical energy.

2. Description of the Related Art

[0002] Temporary storage of energy is becoming increasingly important. Many renewable sources of electric energy, such as wind energy and solar energy, are dependent on atmospheric conditions (wind, sunshine). At times the amount of electricity generated exceeds the demand. At other times the demand exceeds the supply. These imbalances create a need for temporary storage of electric energy. Similarly, the development of electric v ehicles and hybrid vehicles rely on the ability to temporarily store electric energy. [0003] At present batteries arc the devices of choice for temporarily storing electric energy. Batteries store electric energy in the form of chemical energy, i.e.. electric energy is used to drive a chemical conversion inside the battery. On demand the battery releases the stored energy in the form of electric energy.

[0004] Known batteries have a number of wel l-documented disadvantages. Batteries tend to be large and heavy; their efficiencies tend to be poor; and they tend to lose charge over time even when no electric energy is being taken out. Batteries also suffer from the limitation that they can only store energy that is delivered in the form of electric energy, and they can release the stored energy only in the form of electric energy.

[0005] Our earlier patent application No. PCT/EP2012/056582, filed on April 1 1 , 201 2, addresses these shortcomings by providing an apparatus for converting excess energy to chemical energy, in the form of a liquid fuel. The preferred liquid fuel is methanol, because it can be synthesized from readily av ailable reactants (water and carbon dioxide). Methanol has a far greater energy density than do battery-stored electric energy and hydrogen. Moreover, storing methanol requires a simple storage vessel, whereas the storage of electric energy requires an expensive (and heavy) battery, while storage of hydrogen requires a compressor and a pressure vessel .

[0006] The conversion of excess energy to a liquid fuel is particularly attractive if the energy is to be used at a different location than where it is generated, or for propelling a vehicle, for example. This is because of the high energy density of l iquid fuels. As explained in the above cited patent appl ication the liquid fuel can be stored at the location where it is generated, for conversion back to electric energy at a later time. This is an attractive option for longer term energy storage, for example for storing solar energy generated during the summer for heating during the winter. It has however been found that the conversion of excess energy to a liquid fuel followed by conversion back to electric energy inev itably results in significant losses. For this reason it may be more cost effective to store the electric energy in some type of battery if the electric energy is intended for to be used in the near future, for example excess solar energy stored during the daytime for use after sunset.

[0007] Thus, there is a particular need for a device that stores energy, and offers the option of releasing the stored energy in different forms.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention addresses these problems by providing a device for energy storage and conversion, containing a chemical energy cache that is capable of absorbing energy and either elcctrochemically storing the absorbed energy or el ect ro-cat a 1 y t i cal 1 y transferring the absorbed energy to a reactant.

BR IEF DESCRIPTION OF THE DRAWINGS

[0009] The features and advantages of the invention will be appreciated upon reference to the following draw ings, in w hich: [0010] FIG. 1 illustrates the basic concept of the device of the present invention is so-called battery mode;

[0011] FIG. 2 illustrates the basic concept of the device of the present invention is so-called reactor mode; [0012] FIG. 3 illustrates the basic concept of the device of the present invention with a two- step reactor for synthesizing methanol;

[0013] FIG. 4 illustrates the basic concept of the device of the present invention with a one- step reactor for synthesizing methanol ; [0014] FIG 5 illustrates a specific embodiment of the device of the present invention.

[0015] FIG 6 shows operation of an electrochemical reactor in the conversion of carbon dioxide to methanol.

[0016] FIG 7 shows the reactor of Figure 6 when operated as a storage battery.

[0017] FIG 8 shows the battery of figure 7 in discharge mode. [0018] FIG 9 shows an alternate embodiment of the electrochemical reactor.

[0019] FIG 10 illustrates an alternate embodiment wherein the system comprises multiple electrode.

DETAILED DESCRIPTION OF THE INVENTION [0020] The following is a detailed description of the invention. Definitions

[0021] The term "chemical energy cache" as used herein means a chemical compound or a combination of interacting chemical compounds capable of absorbing and storing energy. Examples of single compound chemical energy caches include fluorescent and

phosphorescent materials, which absorb light energy and release it in the form of light.

Certain solids absorb heat energy by converting to a different crystal structure, and release the absorbed heat energy while reverting back to the original crystal structure.

[0022] Examples of combinations of interacting chemical compounds include redox couples, which comprise a reductant and an oxidant. Redox couples absorb energy when the reductant is oxidized and the oxidant is reduced, and v.v.

[0023] Examples of chemical energy caches that are part icularly suitable for use in the present invention include organic Ionic Liquids and inorganic molten salts and inorganic molten salt hydrates. [0024] The term "organic Ionic Liquids" as used herein refers to compounds comprising a cation and an anion, of which at least the cation is an organic molecule. Organic Ionic Liquids are liquid at relatively low temperatures, having melting points, for example, at temperatures below 100°C. Examples include l-ethyi-3-methylimidazoiium tetrafluoroborate; l-butyl-3-methylimidazoiium bis(trifluoromethyisulfonyl)imide. Further examples are described in WO 03/029329 A2, the disclosures of which are incorporated herein by reference.

[0025] The term "inorganic molten salt" as used herein refers to inorganic compounds comprising a metal cation and an inorganic anion, such as hal ide, carbonate, sulfate, and the like. Combinations of a molten metal and a molten salt containing a cation of the same metal are particularly useful as chemical energy caches for use in the present invention. Specific examples include halides of sodium, potassium and l ithium, particularly the eutectic mixture of lithium chloride and potassium chloride; chromates of alkal ine earth metals, in particular calcium chromate and barium chromate; double salts, such as sodium aluminum chloride ( aA!CLi), which has a conveniently low melting point (157 °C); a mixture of MgCi ₂ KG and NaCl, etc.

[0026] The term "inorganic molten salt hydrate ^" as used herein refers to hydrated inorganic salts that, as a result of the hydration, have a much lower melting point than the

corresponding anhydrous salts. Inorganic molten salt hydrates have properties that are very similar to those of organic Ionic Liquids. Inorganic molten salt hydrates offer important advantages over organic Ionic Liquids in that they are far less expensive, and in general chemically and thermally more stable. In addition, organic Ionic Liquids generally require substantially water-free conditions, whereas inorganic molten salt hydrates are tolerant of moisture, even in excess of the amount of water of hydration. On the other hand, organic Ionic Liquids can be made to meet very specific requirements.

[0027] Examples include hydrates of the halides of alkal i and alkaline earth metals, such as Ltd. MgCK and ZnC12. Molten salt hydrates differ from normal aqueous solutions in that all the water is tightly bound within the inner hydration sphere of the cation. ZnCi ₂ .4H ₂ 0 is particularly preferred. [0028] The term "ground state" as used herein refers to the chemical energy cache in a state in which it contains l ittle or no stored energy. Any energy it may have absorbed at an earlier time has been released or dissipated, and the chemical energy cache is available to absorb its full capacity of energy.

[0029] The terms "excited state" and "charged state" refer to the chemical energy cache in a state in which it stores a significant amount of releasablc energy. These terms are used interchangeably, with the proviso that if the stored energy is electric energy one w ill be inclined to refer to that state as "charged", whereas if the stored energy is electromagnetic energy one will be incl ined to refer to that state as "excited", in line w ith the customary usage of these terms. It will be understood, however, that one of the aspects of the device of the present invention is the interehangeability of different forms of energy, which is why the terms "excited state" and "charged state" can be used interchangeably as well, and generally mean a state other than the ground state.

[0030] In its broadest aspect the present invention relates to a device for energy storage and conversion, containing a chemical energy cache that is capable of absorbing energy and cither electrochemically storing the absorbed energy or electro-catal ytical ly transferring the absorbed energy to a reactant. The main advantages of this dev ice are that if offers the flexibility of storing energy for subsequent release as electric energy, or for a chemical conversion to, for example, a liquid fuel.

[0031] The chemical energy cache can be selected to absorb one or more of a variety of energy forms, for example heat energy, electric energy, electromagnetic radiation, nuclear radiation, or a combination thereof. Examples of electromagnetic radiation include infrared radiation, visible light, u.v., microwave, and X-ray.

[0032] In one embodiment the absorbed energy is electric energy, for example from a renewable resource such as solar energy or wind energy.

[0033] The chemical energy cache can be a solid or a liquid. Suitable examples of liquid chemical energy caches include organic Ionic Liquids; inorganic molten salts; and inorganic molten salt hydrates.

[0034] In one embodiment the device is capable of releasing stored energy in the form of electric energy. In another embodiment the chemical energy cache in the dev ice is capable of el ect ro-catal yt ical I y transferring stored energy to a reactant, thereby catalyzing conversion of the reactant to a reaction product. The reaction product can be a liquid fuel . [0035] In an embodiment a reactant is water, and the reaction product is hydrogen. In another embodiment the reactant is carbon dioxide, and the reaction product is carbon monoxide.

[0036] In a preferred embodiment one or two devices electrocatal yt ical I y transfer energy to water and carbon dioxide, simultaneously forming hydrogen and carbon monoxide

("syngas"). The syngas mixture can be converted to, e.g., methane, methanol, Fischer- Tropsch liquid alkanes, and the like, using existing technologies.

[0037] Carbon dioxide for use as a reactant can be generated by the combustion of a carbon-containing fuel, such as biomass or a fossil fuel. Carbon dioxide can also be produced by selectively adsorbing carbon dioxide from atmospheric air, and subsequently desorbing the adsorbed carbon dioxide.

[0038] Water for use as a reactant can similarly be "harvested" from atmospheric air by selective adsorption followed by desorption.

[0039] DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS/EXAMPLES [0040] The following is a descri tion of certain embodiments of the invention, given by way of example only and with reference to the drawings. Referring to FIG. 1 , the basic concept is shown with the device of the invention in "battery mode." Energy in the form of electrons (electric energy), photons ( infrared, visible light or U.V.) or electro-magnetic waves (microwave energy. X-ray) is suppl ied to the dev ice in the ground state or discharged state. [0041] As a result of absorption of the supplied energy, th e device adopts an excited or charged state, as depicted by the second box. After a certain amount of time has elapsed, the device is still in the excited or charged state, as depicted by the thi d box. At this point energy- is removed from the device in the form of electrical power. After the stored energy is depleted the device is back in its ground state, and once again ready to absorb energy. [0042] Figure 2 shows the device in its reactor mode. The absorption and storage of energy are as described for Figure 1 . While in the excited or charged state the device receiv es reactants, such as water, carbon dioxide, etc. These reactants are converted to reaction products, such as hydrogen and carbon monoxide. Reaction products are removed from the device, either as primary reaction products (hydrogen, carbon monox ide), or as secondary reaction products (methanol ). The device converts back to its ground state upon transfer of the stored energy to the reactants.

[0043] Figure 3 shows a dev ice with a two-step reactor mode. In the first step water is converted to hydrogen (as wel l as oxygen ); in the second step carbon diox ide is reacted with hydrogen to form methanol .

[0044] Figure 4 shows a one-step reactor mode. Water and carbon dioxide are

simultaneously fed to the device, and methanol is obtained as the reaction product.

[0045] Figure 5 show s a specific embodiment of the invention. The dev ice contains organic Ionic Liquid and/or inorganic molten salt as the chemical energy cache. The device also contains a catalyst, such as a Ni catalyst. The catalyst is present in the form of two electrodes, a cathode and an anode (the cathode is shown schematically as one catalyst particle). Water and carbon dioxide are supplied as reactants. Energy is supplied in the form of electric energy from an array of photovoltaic cells. The half reaction taking place at the cathode (reduction of water to hydrogen ) is depicted schematically. The other half reaction is the reduction of carbon dioxide to carbon monox ide. At the anode oxygen is formed (not shown ). Hydrogen and carbon monoxide react catalytical ly to form methanol.

[0046] Figure 6 show s an electrochemical reactor 1 comprising a vessel 30 filled w ith the zinc chloride solvent 31 . Immersed in solvent 31 are a first electrode 10 and a second electrode 20. An electric potential 32 is applied to electrodes 10 and 20 so that electrode 10 acts as a cathode and electrode 20 as an anode. Anode 20 is surrounded by an electrically insulating, proton-permeable membrane 21 of the kind routinely employed in fuel cel ls. Nation® from Dupont is an example of a suitable membrane material. Anode 20 preferably comprises manganese (III) oxide (Mn^Ch )- Alternately anode 20 may comprise an Ag ₂ 0/Ag redox system. [0047] Electrode 10 has the form of a hol low, porous tube, made of an electrical ly conducting material, for example a porous metal ; a hollow porous graphite rod; a hollow- graphite honeycomb rod, or the l ike.

[0048] Carbon dioxide gas is supplied to cathode 10 under an overpressure that is high enough to force carbon diox ide gas through the cathode material into the solvent, yet not so high as to cause excessive formation of carbon dioxide bubbles on the surface of cathode 1 0. As shown in Figure 6, water is supplied to anode 20.

[0049] During operation of the reactor the reaction at the anode is the half reaction of the well-known water electrolysis reaction : 6H ₂ 0→ 4H ₃ 0 ⁺ + 4c + 0 ₂ (1)

[0050] Although chloride ions arc present in the medium, the formation of molecular chlorine is suppressed due to the presence of manganese (III) oxide in the anode.

[0051] Several competing reactions can take place at the cathode:

C0 ₂ + 6H ₃ 0 ⁺ + 6e → CH ₃ OH + 7H ₂ 0 (2) C0 ₂ + 2l O + e → CO + 3H ₂ 0 (3)

2H ₃ 0 ⁺ + 2e ^~ → H ₂ + 2H ₂ 0 (4)

Zn ² + 2e → Zn (5 )

[0052] In general reaction (2) is the most desirable, as it results in the formation of a liquid fuel . Reaction (2) can be promoted by incorporating a methanol formation catalyst in cathode 10, such as nickel. The presence of nickel may also promote conversion of carbon monoxide (from reaction (3)) and hydrogen ( from reaction (4) to methanol. Reaction (5) is favored if the medium is proton depleted. Reaction (5 ) can be suppressed by increasing the proton concentration of the reaction mixture, for example by the addition of a mineral acid.

Hydrochloric acid is the preferred mineral acid. [0053] The overall reaction equation for reactor 1 is:

4 H ₂ 0 + 2 C0 ₂ → 2 CH ₃ OH + 3 0 ₂ (6)

[0054] Figure 7 shows the operation of the reactor as a storage battery for electric energy. An electric voltage 32 is suppl ied to the electrodes, as in Figure 6. No reactants are suppl ied to the electrodes, however. In this mode the reaction at cathode 1 0 is reaction (5 ):

Zn ^{2 '} + 2e → Zn (5 )

The reaction at anode 20 is oxidation of Mn(III) to Mn(IV): Mn ₂ 0 ₃ + 2 0ff → 2 Mn0 ₂ + H ₂ 0 + 2c ^" and the overall reaction equation:

Zn ²⁺ + Mn ₂ 0 ₃ + 2 Off → Zn + 2 Mn0 ₂ + H ₂ 0

[0055] With an anode comprising silver the reaction at the anode

[0056] 2Ag + H ₂ 0 -» Ag ₂ 0 + 2H + 2e and the overall reaction is then:

ZnCl ₂ + 2Ag + H ₂ 0 -» Ag ₂ 0 + Zn + 2HC1

[0057] In an acidic environment the reaction equations are:

Zn ² + 2e ^~ → Zn

Mn ₂ 0 ₃ + 3 H ₂ 0 → 2 Mn0 ₂ + 2 I hO + 2c ^" and the overall reaction :

Zn ²⁺ + Mn ₂ 0 ₃ + 3 H ₂ 0 → 2 Mn0 ₂ + 2 H ₃ 0 ⁺ + Zn

[0058] Figure 8 shows the reactor of Figure 1 operated as a charged battery in the process of being discharged. Al l reactions are identical to those of Figure 7, except that the reactions proceed in opposite direction. The overall reaction in an acidic environment is:

2 Mn0 ₂ + 2 H _. ,0 ^" + Zn → Zn ²⁺ + Mn ₂ 0 ₃ + 3 H ₂ 0 (9a)

With silver present in the anode the overall reaction can be:

[0059] Ag ₂ 0 + Zn + 2HC1 -» ZnCi ₂ + 2Ag + H ₂ 0 (9b)

[0060] It will be understood that the reactions of Figure 6 can be conducted in the charged battery of Figure 8. In this embodiment no external electric energy needs to be suppl ied. Instead, electric energy stored in the reactor 1 is used to drive a reaction such as given by equation (6).

With silver present in the anode the overall reaction for this mode can be written as:

Ag ₂ 0 + 3Zn + 6HC1 + C0 ₂ ->3Z.nCl ₂ + 4Ag + CH ₃ OH + H ₂ 0 + 0 ₂ (9c) [0061] Figure 9 shows a second embodiment of the electrochemical reactor. The electrically insulating membrane is placed intermediate the cathode and the anode. The electrolyte surrounding the anode is different from the electrolyte surrounding the cathode, the former comprising, for example, MnS0 ₄ and/or H ₂ S0 ₄ . In reactor mode the reactions taking place are as described above in reaction equations (1) through (6).

[0062] In energy storage mode the cathode reaction is given by reaction equation (5). The reaction at the anode involves oxidation of dissolved Mn(II) to solid Mn( IV), as follows:

MnSO i + 2H ₂ 0 -» Mn0 ₂ + H ₂ S0 ₄ + 2H + 2e (10)

The overall reaction is: ZnCi ₂ + MnS0 ₄ + 2H ₂ 0 - Mn0 ₂ + H ₂ S0 ₄ + Zn + 2HC1 (1 1)

[0063] In a battery discharge mode Mn(IV) is reduced to Mn(III), resulting in overall reaction as per equation (12):

2Mn0 ₂ + Zn + 2HC1 -» ZnCi ₂ + Mn ₂ 0 ₃ + H ₂ 0 ( 12 )

[0064] As in the previous embodiment, stored electrical energy can be used in the

2

conversion of carbon dioxide, with ox idation of metall ic Zn to Zn ^' and reduction of n( IV) to solid Mn( l l l ) or dissolved Mn(II), for example:

2Mn0 ₂ + Zn + 2HC1 + 2C0 ₂ + 3H ₂ 0 -» ZnCi ₂ + Mn ₂ 0 ₃ + 2CH30H + 30 ₂ (13)

[0065] In an alternative embodiment of the invention, the location where the electrical energy is stored and where the energy is used to perform chemical conversions are separated. This embodiment is illustrated at the hand of figure 10. In this figure it is shown that the system can consist of multiple electrodes. In this example the combination of the first two electrodes 40 and 4 1 , by closing sw itch 43, allows energy to be stored by making an oxygen evolution reaction occur in electrolyte 45 at electrode 40, while at the same time making zinc deposition occur from electrolyte 46 on electrode 41 , according to reactions (1) and (5 ). In order to utilize the stored energy for chemical conversions, switch 44 can be closed to connect electrodes 41 and 42. The previously deposited zinc will go back onto solution, according to the reverse of reaction (5). The energy released by this dissolution reaction can be used at electrode 42 to drive any of the reactions (2), (3), (4), (6) or combinations thereof. [0066] It will be understood that the electrolytes 45, 46 and 47 can be of the nature as described previously in this patent, and that they can be, depending on their nature and composition, separated by membranes 48 and/or 49.

Further embodiments can include more electrode/electrolyte units, for example for the recovery of the stored energy in the form of electricity, similarly to a zinc-air battery.

[0067] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. For example, the reaction may be modified by changing the rcactants and/or the catalysts. [0068] Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.