# Patent

## Title
Hybrid metal oxide cycle water splitting

## Abstract

Hybrid thermochemical water splitting cycles are provided in which thermally reduced metal oxides particles are used to displace some but not all of the electrical requirements in a water splitting electrolytic cell. In these hybrid cycles, the thermal reduction temperature is significantly reduced compared to two-step metal-oxide thermochemical cycles in which only thermal energy is required to produce hydrogen from water. Also, unlike the conventional higher temperature cycles where the reduction step must be carried out under reduced oxygen pressure, the reduction step in the proposed hybrid cycles can be carried out in air, allowing for thermal input by a solar power tower with a windowless, cavity receiver.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority to U.S. provisional application Ser. No. 61/368,756, filed on Jul. 29, 2010, which is hereby incorporated by reference in its entirety.


BACKGROUND OF THE DISCLOSURE
The present disclosure relates generally to methods and apparatus associated with hybrid metal oxide cycle water splitting.
SUMMARY OF THE INVENTION
In one aspect of the disclosure, a hea hybrid thermochemical water splitting cycle includes thermally reducing a metal oxide with concentrated solar energy and reoxidizing an oxygen-deficient metal oxide in an electrochemical cell, wherein the chemical potential for the reactions is provided by the metal oxide and electrical energy, and wherein the electrical energy required is significantly lower than needed for water or carbon dioxide electrolysis alone.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a schematic view of a hybrid metal oxide cycle; and
FIG. 2 shows one implementation strategy of the hybrid metal oxide cycle of FIG. 1.



DETAILED DESCRIPTION
Hybrid thermochemical water splitting cycles are proposed in which thermally reduced metal oxides particles are used to displace some but not all of the electrical requirements in a water splitting electrolytic cell. In these hybrid cycles, the thermal reduction temperature is significantly reduced compared to two-step metal-oxide thermochemical cycles in which only thermal energy is required to produce hydrogen from water. Also, unlike the conventional higher temperature cycles where the reduction step must be carried out under reduced oxygen pressure, the reduction step in the proposed hybrid cycles can be (but are not required to be) carried out in air, allowing for thermal input by a solar power tower with a windowless, cavity receiver. These proposed hybrid cycles could also potentially enable the use of heat from a nuclear reactor. Regardless of the energy input source, solar or nuclear, in the hybrid scheme the products from thermal reduction are utilized as a chemically active anode material in a water splitting electrolyzer. Because the anode is re-oxidized during electrolysis, the electrical input required to split water is reduced substantially below the electric input required to drive a conventional water splitting electrolyzer. For example, the ideal decomposition voltage can drop from the 1.23 volts needed in a conventional cell to values as low as 0.23 volts. This approach appears well suited to an electrolyzer where the anode is a packed or moving (possibly fluidized) bed of particles. The bed could either be stand-alone, or could be a mixture of a conductor, e.g. Ni metal, and the solar reduced oxide. This cell design concept could be integrated with a reactive-particle solid particle receiver. Appropriate sizing of the receiver relative to the electrolyzer would allow for round-the-clock continuous operation of the electrolyzer.
Thermochemical cycles are an attractive alternative to electrolysis as they have the potential for higher energy efficiency due to the fact that they avoid the need to first convert thermal energy into electrical energy. There are many proposed metal oxide thermochemical cycles for water splitting. For example, the Iron Oxide cycle is one that has been interesting to the research community because the reduced iron oxide is capable of reacting with water to produce hydrogen. However, in this cycle very high temperatures are needed to thermally reduce magnetite, Fe3O4, to the reduced oxide, wustite, FeO. Complicating matters is the fact that the recovery of FeO is dependent on the reduction step being conducted under reduced oxygen partial pressure, e.g. in a vacuum or in the presence of an inert sweep gas. Also, the oxides melt and are somewhat volatile at the required temperatures. The thermodynamic high temperature requirements and thermodynamic and kinetic requirements forcing the reaction to take place under an inert atmosphere as well as numerous materials challenges posed for these types of cycles compromise significantly the potential of engineering these cycles into an efficient and industrially economically realizable process. We are, therefore, interested in processes where the thermal reduction step can be done at temperatures below 1500° C. and even in air.
As an example, oxygen can be driven off from cobalt and manganese oxides (i.e. Co3O4 Mn2O2, and Mn2O3) at significantly lower temperatures than needed to thermally reduce magnetite. In addition, oxygen can also be produced from hematite, Fe2O3, at much lower temperatures.

3Fe2O3→2Fe3O4+1⁄2O2  (1)

The factor previously eliminating the use of these attractive thermal reduction chemistries has been the other half of the two-step cycle. The Gibbs free energy, ΔG, of the oxidation Fe3O4 and the thermally reduced states of cobalt and manganese oxides with water are positive at any temperature above and including room temperature. That is, they are thermodynamically stable in the presence of water and steam, and will not spontaneously oxidize to yield hydrogen. (Note: Because the reaction with water is not favorable, steam is essentially inert in the system and could be employed gainfully as a sweep to promote the reduction reaction if desired).
The factor previously eliminating the use of these attractive thermal reduction chemistries has been the other half of the two-step cycle. The Gibbs free energy, ΔG, of the oxidation Fe3O4 and the thermally reduced states of cobalt and manganese oxides with water are positive at any temperature above and including room temperature. That is, they are thermodynamically stable in the presence of water and steam, and will not spontaneously oxidize to yield hydrogen. (Note: Because the reaction with water is not favorable, steam is essentially inert in the system and could be employed gainfully as a sweep to promote the reduction reaction if desired).

2Fe3O4+H2O(g)→3Fe2O3+H2(g) ΔG>0  (2)

Employing a third reaction with sodium hydroxide or potassium hydroxide has been proposed to make the oxidation thermodynamics favorable for metal oxide cycles in which the oxidation reaction is not favorable However, the addition of the third reaction step and the addition of hydroxides significantly complicate the cycles. A key insight is that these oxidation reactions can be made favorable and the complications of a third step eliminated with the addition of only a small applied voltage at ambient temperature, one significantly less than that required for conventional electrolysis.

2Fe3O4+H2O(g)+Power(IV)→3Fe2O3+H2(g) ΔG<0  (3)

Comparison to Alternate Approaches:
The proposed hybrid cycle 10, shown schematically in FIG. 1, is similar to conventional two-step thermochemical cycles with the exception that the oxidation step is electrochemically assisted. In the hybrid thermochemical cycle 10, a metal oxide 12 is thermally reduced (driving off oxygen 16) with concentrated solar energy 18. The oxygen deficient metal oxide 14 is subsequently reoxidized in an electrochemical cell 20 to reduce water to hydrogen or carbon dioxide to carbon monoxide. Heat 20 extracted from the reduced metal oxide 14 can be used for power generation 22. The chemical potential for the water or carbon dioxide reducing reactions is provided by the reduced metal oxide and electrical energy. The electrical energy is required is significantly lower than needed for water or carbon dioxide electrolysis alone. The payoff for this hybrid approach is that the extreme challenges posed by the ultra-high temperature reduction (e.g. materials degradation, volatilization, limited heat source options, thermal energy management, closed systems with inert gas sweeps, etc.) are exchanged for a simpler electrochemical step. Because the hybrid cycle 10 is significantly more straight forward, it should be much easier to implement and thus in practice should be more efficient than the proposed processes that split water with an exothermic reaction like that corresponding the iron oxide cycle.
Note that in the hybrid approach, the reduction and oxidation steps of two step metal oxide cycles are separated and can be performed independently of each other. An advantage of this is that while the solar driven thermochemical step can be carried out when the sun is available, the electrochemically driven oxidation step can occur when convenient. Therefore, by over-sizing the thermally driven reduction process relative to the electrochemical oxidation process, it is possible to achieve round-the-clock operation. This has the benefit of maximizing the utilization of the electrochemical equipment and minimizing issues associated with starting and stopping the electrochemical reactor.
It may be noted that this metal-oxide hybrid approach is analogous to the Hybrid Sulfur cycle where the hydrogen production step is electrochemical vs. the 3-step Sulfur-Iodine thermochemical cycle. However, the metal oxide approach utilizes no corrosive or hazardous chemicals and utilizes only gas-solid and gas-liquid reactions, making for convenient separations of valuable products from the working materials. It is also worth pointing out that the hybrid process is an alternative to solar thermal electrolysis processes that have been explored over the past thirty years. They were motivated by the fact that the higher the operating temperature of an electrolytic cell the more one could substitute sunlight for valuable electric work. Thus a number of studies show running modestly high solar processes where sunlight and electric energy is simultaneously supplied to the solar reactor. Our hybrid metal oxide cycle 10 is thermodynamically equivalent to this approach, but by separating the electrolytic step from the thermal reduction step, one greatly reduces the complexity of the solar reactor and allows for a quasi-24-hour continuous solar process. The thermodynamics of metal oxide cycles allow for a much higher percentage of thermal vs. electrical contribution to water splitting at lower temperatures, compared to direct high-temperature water electrolysis.
Materials and Thermodynamics:
A non-exclusive list of metal oxides we believe are of interest for this hybrid approach includes Fe2O3, MnO2, Mn2O3, and Co3O4 where the products from the solar reduction step are Fe3O4, MnO, and CoO. By selecting the metal oxides, or perhaps mixed metal oxides, it may be possible to tailor the thermodynamics to a particular temperature range, perhaps enabling application to nuclear power sources or solar power towers. Thus mixed metal oxides, e.g. ferrites of cobalt, nickel, manganese, etc. and similar materials are also of interest as are other families of materials metals with numerous possible valence states such as vanadium, molybdenum, niobium, chrome, tungsten, and cerium-oxides.
Tables 1 and 2 show the thermodynamics of water splitting with a hybrid iron oxide (hematite) cycle. Table 1a showing the reduction of magnetite is provided as a baseline comparison. Clearly, the thermal reduction of hematite is far easier to accomplish. Table 2 shows that only about 10.6 kcal would be needed in a hematite/magnetite hybrid cell where the anode is chemically active whereas about 56.7 kcal of electrical energy is needed to electrochemically split water at room temperature with the traditional inert anode used in electrolyzers; the minimum voltage is reduced to about 0.23 Volts from 1.23 Volts. Tables 3 and 4 show similar calculations with cobalt oxide and Tables 5 and 6 show calculations for one particular manganese oxide couple. For cobalt, the thermal reduction temperature is significantly reduced while the cell voltage is 0.38 Volts, which is still significantly less than needed to electrolyze water directly. For the manganese oxide couple the cell voltage is also about 0.38 volts.







TABLE 1







Thermodynamics of hematite reduction 3Fe2O3 = 2Fe3O4 + 1⁄2O2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
55.509
31.723
46.844



100.000
55.676
32.213
43.656



200.000
55.864
32.666
40.408



300.000
55.976
32.879
37.131



400.000
56.365
33.495
33.818



500.000
57.543
35.109
30.398



600.000
59.194
37.144
26.761



700.000
57.776
35.614
23.118



800.000
57.708
35.545
19.562



900.000
57.565
35.420
16.012



1000.000
57.220
35.139
12.483



1100.000
56.769
34.798
8.986



1200.000
56.286
34.459
5.524



1300.000
55.833
34.161
2.093



1400.000
55.459
33.930
−1.311

















TABLE 1a







Thermodynamics of magnetite reduction Fe3O4 = 3FeO + 1⁄2O2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
75.541
30.449
67.224



200.000
75.483
30.508
61.048



400.000
74.380
28.626
55.110



600.000
71.304
24.670
49.763



800.000
70.484
23.800
44.943



1000.000
70.619
23.908
40.181



1200.000
71.130
24.279
35.363



1400.000
88.982
35.116
30.228



1600.000
56.815
17.941
23.208



1800.000
57.297
18.186
19.595



2000.000
57.789
18.412
15.935

















TABLE 2







Thermodynamics of water dissociation with magnetite


2Fe3O4 + H2O = 3Fe2O3 + H2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
14.431
13.226
10.818



100.000
12.068
5.108
10.161



200.000
11.064
2.722
9.775



300.000
9.895
0.496
9.611



400.000
7.844
−2.786
9.719



500.000
4.961
−6.762
10.189



600.000
1.621
−10.851
11.096



700.000
1.367
−11.134
12.202



800.000
−0.223
−12.687
13.393



900.000
−1.723
−14.026
14.732



1000.000
−3.008
−15.078
16.189

















TABLE 3







Thermodynamics of cobalt oxide reduction Co3O4 = 3CoO + 1⁄2O2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
46.532
33.919
37.267



100.000
47.737
37.726
33.659



200.000
48.516
39.590
29.784



300.000
49.020
40.564
25.770



400.000
49.299
41.017
21.688



500.000
49.365
41.112
17.579



600.000
49.214
40.932
13.475



700.000
48.837
40.525
9.400



800.000
48.218
39.922
5.376



900.000
47.343
39.143
1.422



1000.000
46.193
38.204
−2.447



1100.000
44.750
37.115
−6.214



1200.000
42.997
35.884
−9.865



1300.000
40.914
34.517
−13.387



1400.000
38.483
33.020
−16.765

















TABLE 4







Thermodynamics of water dissociation with


3CoO + H2O(g) = Co3O4 + H2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
11.208
−23.507
17.629



100.000
10.238
−26.580
20.156



200.000
9.684
−27.908
22.889



300.000
9.396
−28.469
25.712



400.000
9.323
−28.590
28.568



500.000
9.453
−28.413
31.421



600.000
9.786
−28.010
34.243



700.000
10.332
−27.420
37.016



800.000
11.103
−26.668
39.722



900.000
12.114
−25.769
42.345



1000.000
13.382
−24.734
44.871

















TABLE 5







Thermodynamics of manganese oxide reduction


2Mn2O3 = 4MnO + O2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
44.529
26.551
37.277



100.000
44.597
26.773
34.607



200.000
44.571
26.713
31.931



300.000
44.493
26.566
29.267



400.000
44.379
26.382
26.619



500.000
44.231
26.178
23.991



600.000
44.049
25.957
21.384



700.000
43.829
25.720
18.800



800.000
43.570
25.466
16.241



900.000
43.266
25.195
13.708



1000.000
42.913
24.907
11.203



1100.000
42.509
24.602
8.727



1200.000
42.048
24.278
6.283



1300.000
41.527
23.936
3.872



1400.000
40.942
23.576
1.496



1500.000
40.289
23.197
−0.843

















TABLE 6







Thermodynamics of water dissociation with


2MnO + H2O(g) = Mn2O3 + H2(g)












T, ° C.
deltaH, kcal
deltaS, cal/K
deltaG, kcal
















0.000
13.210
−16.139
17.618



100.000
13.378
−15.626
19.208



200.000
13.629
−15.031
20.741



300.000
13.922
−14.470
22.216



400.000
14.243
−13.955
23.637



500.000
14.587
−13.479
25.008



600.000
14.952
−13.036
26.334



700.000
15.339
−12.615
27.616



800.000
15.752
−12.212
28.857



900.000
16.191
−11.821
30.059



1000.000
16.661
−11.437
31.222










Example Implementation Strategy:
An example implementation strategy 40 is shown in FIG. 2. The oxygen producing step in the hybrid metal oxide cycle is thermally driven. A preferred method could be in a reactive particle receiver 42. In the reactive particle solar receiver 42, the oxidized particles would be directly heated and thermally reduced in an open cavity 44, potentially in air. The reactive particle solar receiver 42 is similar to the solid particle receiver except that the particles undergo thermal reduction chemical reactions in the receiver. The particles could be re-circulated within the receiver to increase residence and heat transfer times. The use of inert gas such a nitrogen (it would not require high purity) inside the reactive particle receiver/reactor cavity 46 could further drive the reaction and/or lower the temperatures required. Countercurrent flow of the metal oxide particles and inert gas in the down comer 48 of the reactive particle receiver 42 would be advantageous for driving the thermal reduction reaction with some of the sensible heat in the particles. The hot (reduced) metal oxides could be used to preheat the metal oxide feed. As shown in FIG. 2, an alternative would be to use the sensible thermal energy to drive electricity producing Rankine or other thermodynamic cycle before being fed to the electrochemical cell. Low-grade steam exiting the Rankine cycle could also be employed as an inert sweep in the reactive solid particle receiver.
We envision that the reduced particles would be fed into the electrolyzer as a slurry in water. The slurry may also contain an electrolyte such as potassium hydroxide. This electrochemical step may be facilitated by packed bed or fluidized bed electrodes, an approach that has received a significant amount of research. In this concept the reduced metal oxide, mixed or doped with a kinetically oxygen-inert electrically conductive material, such as Ni, would be the cell's anode. The metal oxide is the active anode and the Ni is facilitating the needed electron transfer for the anodic reaction to take place, but is not chemically participating in the reaction. The required electrochemical potential needed to split water is reduced in comparison to the required potential of a traditional electrolysis of water by the chemical potential from the metal oxide. The active anode, however, is consumed as it is oxidized during the electrolysis process and would be circulated through the electrolytic cell. After leaving the electrolytic cell the reoxidized metal oxide particles would be rinsed to remove the electrolyte and dried before returning to the reactive particle receiver.
Other Embodiments
Other analogous redox reactions are candidates for a similar approach. For example, the splitting of CO2 to CO and O2 should be amenable to same approach and materials outlined here.
The hybrid electrochemical approach could also be used to enhance the oxidation kinetics of systems wherein the reactions are thermodynamically favorable, but are reaction rate limited.
It is contemplated that the parts and features of any one of the specific embodiments described can be interchanged with the parts and features of any other of the embodiments without departing from the spirit and scope of the present disclosure. The foregoing description discloses and describes merely exemplary embodiments of the present disclosure and is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. As will be understood by those skilled in the art, the disclosure may be embodied in other specific forms, or modified or varied in light of the above teachings, without departing from the spirit, novelty or essential characteristics of the present disclosure. Accordingly, the disclosed embodiments are intended to be illustrative, but not limiting, of the scope of the invention. The exclusive right to all modifications within the scope of this disclosure is reserved.

## Claims

1. A hybrid thermochemical water splitting cycle comprising the steps of:
thermally reducing a metal oxide with concentrated solar energy to produce an oxygen-deficient metal oxide;
feeding the oxygen-deficient metal oxide to an electrochemical cell; and
reoxidizing the oxygen-deficient metal oxide in the electrochemical cell;
wherein the chemical potential for the reactors is provided by the metal oxide and electrical energy; and
wherein the electrical energy required is significantly lower than needed for water or carbon dioxide electrolysis alone.

2. The hybrid thermochemical water splitting cycle of claim 1, wherein heat extracted from the reduced metal oxide is used for power generation.

3. The hybrid thermochemical water splitting cycle of claim 2, wherein the step of reoxidizing an oxygen-deficient metal oxide in an electrochemical cell reduces water to hydrogen.

4. The hybrid thermochemical water splitting cycle of claim 3, wherein the step of reoxidizing an oxygen-deficient metal oxide in an electrochemical cell reduces carbon dioxide to carbon monoxide.

5. The hybrid thermochemical water splitting cycle of claim 4, wherein the thermal reduction step is carried out in air.

6. The hybrid thermochemical water splitting cycle of claim 5, wherein the electrical energy required is as low as 0.23 volts.