NL2034721B1

NL2034721B1 - Low temperature thermochemical water splitting

Info

Publication number: NL2034721B1
Application number: NL2034721A
Authority: NL
Inventors: Baban Bansode Atul; Offidani Michele; Ganesh Vaishnavi
Original assignee: Univ Delft Tech
Priority date: 2023-04-28
Filing date: 2023-04-28
Publication date: 2024-11-04
Also published as: WO2024225900A1

Abstract

40 ABSTRACT: 5 The invention provides a method for providing hydrogen using a reactor material (100) comprising a redox material (110), wherein the redox material (110) comprises a nanostructure (1) selected from the group comprising a nanorod (10) and a nanowire (20), and wherein the redox material (110) comprises Z 50 wt% cerium dioxide, and wherein the redox material (110) comprises 1-25 wt% iron oxide, and wherein the method comprises: a 10 reduction stage comprising exposing the redox material (110) to a reduction temperature TRed and a reduction gas (410), Wherein the reduction temperature TRed is selected from the range of Z 300 °C, and Wherein the reduction gas (410) comprises an inert gas; an oxidation stage comprising exposing the redox material (110) to an oxidation gas (420), Wherein the oxidation gas (420) comprises water vapor.

Description

Low temperature thermochemical water splitting

FIELD OF THE INVENTION

The invention relates to a method for providing hydrogen. The invention further relates to a system for providing hydrogen. The invention further relates to a use of a redox material for providing hydrogen from water vapor.

BACKGROUND OF THE INVENTION

Methods for the production of hydrogen are known in the art. For instance,

US20140130415A1 describes a method for preparing a fuel using oxygen-storing compound nanoparticles is provided, in which the nanoparticles is heated at a first temperature to release an amount of oxygen, thereby producing a reduced oxide compound, and the reduced oxide compound is exposed to a gas at a second temperature to produce the fuel. The gas can include carbon dioxide and water vapor, and the fuel can include carbon monoxide and/or hydrogen.

The oxygen-storing compound nanoparticles can be nano ceria or nano ceria doped with one or more metals, such as Cu and/or Zr. US20140130415A1 further discloses a system for carrying out the method.

SUMMARY OF THE INVENTION

Hydrogen gas is currently being investigated as an alternative to natural gas for heating purposes. Apart from this, hydrogen-based fuels may also be used as an alternative to fossil fuels to power machinery. The advantages of hydrogen-based fuels may be their high energy density and the reduced emission of greenhouse gases compared to alternative (non- )fossil fuels. Especially, the main product formed after combustion of hydrogen-based fuels may be water. Furthermore, hydrogen gas may be produced from renewable sources, such as water. Hydrogen production from water may occur upon thermal decomposition, electrolysis, and photo-assisted water splitting.

Another method for hydrogen production from water may be thermochemical water splitting. In thermochemical water splitting, a redox material may be provided to a thermochemical reactor, to reduce the temperature at which water splitting occurs. In such systems, the redox material may first be heated to release oxygen from the material. Then, a (water-comprising) gas may be provided to the reduced material at a second temperature to generate hydrogen gas. Using redox materials, the temperature for water splitting may be reduced to < 1500 °C. The prior art may describe methods for thermochemical water splitting using volatile redox materials. Volatile redox materials may have improved hydrogen production rates compared to non-volatile redox materials. However, volatile redox materials may turn gaseous under elevated temperature. Hence, gas quenching may be performed to separate the gaseous product and the redox material. This may lower the productivity and lifetime of the redox material. Alternatively, (non-volatile) redox nanomaterials may be used.

Redox nanomaterials may have higher hydrogen production rates compared to redox materials, due to an increase in contact area between the redox material and the (water-comprising) gas.

The prior art may describe methods for thermochemical water splitting using redox (nano)materials at elevated temperatures. Yet, under such conditions, nanomaterials may be susceptible to sintering. Sintering may be a process in which particles agglomerate or fuse together upon application of high heat and/or pressure. Sintering may therefore decrease the contact area between the redox material and the (water-comprising) gas. Sintering may further cause degradation of the (redox) nanomaterial, and a reduced hydrogen production. Thus, the sintering of the redox (nano)materials used in the prior art may prevent practical implementations of such approaches.

Additionally or alternatively, the prior art may describe methods for thermochemical water splitting at lower temperatures (e.g. < 800 °C). However, such methods may typically result in poor hydrogen production rates.

Hence, prior art approaches may suffer from a low stability of the redox (nano)material at operational conditions, particularly at operational temperatures, and/or from a low hydrogen production rate.

Thus, it is an aspect of the invention to provide an alternative method for providing hydrogen, which preferably further at least partly obviates one or more of above- described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

According to a first aspect, the invention provides a method for providing hydrogen using a reactor material comprising a redox material. In embodiments, the redox material may comprise a (one-dimensional) nanostructure. Further, in embodiments, the nanostructure may be selected from the group comprising a nanorod and a nanowire.

Additionally or alternatively, in embodiments, the redox material may comprise > 25 wt% cerium dioxide, such as > 40 wt% cerium dioxide, especially > 50 wt% cerium dioxide. Further, in embodiments, the redox material may comprise 0-50 wt% iron oxide, such as 1-40 wt% iron oxide, especially 1-25 wt% iron oxide. In embodiments, the nanostructure, especially the nanorod, or especially the nanowire, may comprise > 25 wt% cerium dioxide, such as > 40 wt% cerium dioxide, especially > 50 wt% cerium dioxide. Further, in embodiments, the nanostructure, especially the nanorod, or especially the nanowire, may comprise 0-50 wt% iron oxide, such as 1-40 wt% iron oxide, especially 1-25 wt% iron oxide. In embodiments, the method may comprise a reduction stage. The reduction stage may comprise exposing the redox material to a reduction temperature Trega and a reduction gas. In embodiments, the reduction temperature Trea may be selected from the range of > 200 °C, such as from the range of > 250 °C, especially from the range of > 300 °C. Furthermore, in embodiments, the reduction gas may comprise an inert gas. Additionally, the method may comprise an oxidation stage. The oxidation stage may comprise exposing the redox material to an oxidation gas. Specifically, in embodiments, the oxidation gas may comprise water vapor.

Hence, in specific embodiments, the invention provides a method for providing hydrogen using a reactor material comprising a redox material, wherein the redox material comprises a nanostructure selected from the group comprising a nanorod and a nanowire, and wherein the redox material comprises > 50 wt% cerium dioxide, and wherein the redox material comprises 1-25 wt% iron oxide, and wherein the method comprises: 1) a reduction stage comprising exposing the redox material to a reduction temperature Tres and a reduction gas, wherein the reduction temperature Tred is selected from the range of > 300 °C, and wherein the reduction gas comprises an inert gas; and ii) an oxidation stage comprising exposing the redox material to an oxidation gas, wherein the oxidation gas comprises water vapor.

Further, in specific embodiments, the (one-dimensional) nanostructure may comprise the nanorod. In such embodiments, the nanorod may comprise > 50 wt% cerium dioxide, wherein the nanorod may comprise 1-25 wt% iron oxide. In particular, in embodiments, the nanostructure, especially the nanorod, may (essentially) have the same composition as the (overall) redox material.

With such a method it may be possible to increase the hydrogen production rate in a thermochemical reactor. Additionally, the use of a (one-dimensional) nanostructure, especially a nanorod, comprising > 50 wt% cerium dioxide may allow hydrogen production at relatively low reactor temperatures. Specifically, the use of a (one-dimensional) nanostructure, especially a nanorod, comprising > 50 wt% cerium dioxide and 1-25 wt% iron oxide may allow hydrogen production at relatively low reactor temperatures. The use of a (relatively) low reactor temperature may extend the lifetime of the redox material. Furthermore, a redox material with > 50 wt% cerium dioxide may be thermally stable at the selected reduction temperatures Tred.

As such, the gas stream provided during the reduction stage and/or oxidation stage may not need to be gas quenched. In embodiments, a redox material further comprising 1-25 wt% iron oxide may show an increased hydrogen production rate compared to redox materials comprising 0 wt% iron oxide, or >25 wt% iron oxide. Hence, in embodiments, the method may provide a relatively high hydrogen production at a relatively low temperature. This may facilitate generating hydrogen at a relatively low energy cost (for a thermochemical water splitting process). Additionally, the redox material comprising the (one-dimensional) nanostructure may be reduced relatively quickly, 1.e., the redox material may facilitate having a relatively short reduction time, which may facilitate increasing cycling between the reduction stage and the oxidation stage, and thus increasing the overall hydrogen production rate. In embodiments, the hydrogen production rate may be defined as the hydrogen production over time, wherein the hydrogen production may be measured in mL Ha per gram of redox material, ml-H;/g.

As indicated above, in embodiments the method may comprise using a reactor material. A reactor material may be a material designed for use in a reactor, wherein a reactor may be a vessel in which chemical reactions may occur, especially a vessel configured to host chemical reactions, such as a batch reactor, a continuous stirred-tank reactor (CSTR), a plug flow reactor (PFR), a semibatch reactor, a catalytic reactor, or a nuclear reactor. Specifically, the reactor may be a thermochemical reactor, such as a furnace reactor, a solar reactor, an infrared reactor, a microwave reactor, a fixed bed reactor, a packed bed reactor, a fluidized bed reactor, a moving bed reactor, a membrane reactor, a plasma reactor, or a flow reactor, though other suitable reactor types will be known to the person skilled in the art. Specifically, the reactor material may participate in the (chemical) reactions and/or processes occurring in the reactor. However, in embodiments, the reactor material may not (directly) participate in the (chemical) reactions and/or processes occurring in the reaction.

In embodiments, the reactor material may comprise a redox material. A redox material may be a material capable of changing its oxidation state. Especially, a redox material may undergo reduction and oxidation reactions. In embodiments, the redox material may lose or gain atoms during reduction and/or oxidation. Additionally or alternatively, the redox material may lose or gain electrons during reduction and/or oxidation. In embodiments, the redox material may change oxidation state under specific stimuli and/or reaction conditions. In embodiments, the redox material may comprise a perovskite or poly-cation oxide. Further, in embodiments, the redox material may comprise a metal oxide. The metal oxide may be selected from the group comprising cerium dioxide, iron oxide, tin oxide, titanium dioxide, zinc oxide,

magnesium oxide, aluminum oxide, silicon dioxide, and zirconium dioxide, though other options will be known to the person skilled in the art.

In further embodiments, the redox material, especially the nanostructure, may comprise > 50 wt% cerium dioxide, such as > 60 wt% cerium dioxide, especially > 70 wt% 5 cerium dioxide. Yet, in embodiments, the redox material, especially the nanostructure, may comprise < 99 wt% cerium dioxide, such as < 95 wt% cerium dioxide, especially < 90 wt% cerium dioxide. Hereafter, cerium dioxide may also be referred to as “CeO:” or “ceria”.

Furthermore, in embodiments, wt% (weight percentage) may refer to a percentage of the total weight. For example, in embodiments, 100 grams of a redox material comprising 50 wt% cerium dioxide may comprise 50 grams of cerium dioxide. In embodiments, the use of > 50 wt% cerium dioxide may have as advantage that the redox material may be thermally stable under conditions suitable for (efficient) reduction and oxidation of the redox material. Further, cerium dioxide may have fast kinetics, shortening the duration of the reduction stage and oxidation stage. This may allow for faster cycling between the reduction stage and the oxidation stage, and subsequently may increase the hydrogen yield per unit time.

In embodiments, the redox material, especially the (one-dimensional) nanostructure, more especially the nanorod, may further comprise a dopant. The dopant may increase the stability and/or the hydrogen production of the redox material and/or the (one- dimensional) nanostructure. Additionally, the dopant may increase the reactivity of the redox material, especially of the (one-dimensional) nanostructure, such that the reduction temperature

Tied may be lowered. In embodiments, the dopant may be selected from the group comprising iron oxide, cobalt oxide, nickel oxide, copper oxide, manganese dioxide, lanthanum oxide, praseodymium oxide, neodymium oxide, samarium oxide, zirconium dioxide, tin oxide, and titanium dioxide, though other dopants will be known to the person skilled in the art. Thus, in embodiments, the dopant may be a transition metal, such as be selected from the group comprising iron oxide, cobalt oxide, nickel oxide, copper oxide, and manganese dioxide.

Further, the dopant may be a rare earth metal, such as selected from the group comprising lanthanum oxide, praseodymium oxide, neodymium oxide, and samarium oxide. Additionally or alternatively, the dopant may be a metal oxide, such as a metal oxide selected from the group comprising zirconium dioxide, tin oxide, and titanium dioxide. In embodiments, the redox material, especially the nanostructure, may comprise the dopant in a concentration range of 0- wt%, such as in the range of 1-35 wt%, especially in the range of 1-25 wt%. Especially, in embodiments, the redox material, especially the (one-dimensional) nanostructure, more especially the nanorod, may comprise 0-50 wt%, such as 1-35 wt%, especially 1-25 wt% iron oxide (as dopant). In specific embodiments, the redox material, especially the nanostructure, may comprise 3-20 wt% iron oxide, such as 7-13 wt% iron oxide. Further, the redox material, especially the (one-dimensional) nanostructure, such as the nanorod, or such as the nanowire, may comprise at least 6 wt% iron oxide, such as at least 7 wt% iron oxide, especially at least 8 wt% iron oxide. Additionally, in embodiments, the redox material, especially the (one- dimensional) nanostructure, such as the nanorod, or such as the nanowire, may comprise at most 14 wt% iron oxide, such as at most 13 wt% iron oxide, especially at most 12 wt% iron oxide. Here, iron oxide may refer to one or more of FeO, Fe30:, FesOs, FesOs, FesO7, Fe25032,

Fe3019, and Fe20:. In embodiments, the addition of iron oxide in 1-25 wt%, especially 6-14 wt%, to a redox material, especially to the nanostructure, comprising cerium dioxide may have an unexpected positive effect compared to a redox material not comprising iron oxide, or a redox material comprising > 25 wt% iron oxide. Here, in embodiments, the positive effect may comprise one or more of a) an increased hydrogen production, b) an increased stability, and c) a higher reactivity.

Further, in embodiments, the redox material, especially the (one-dimensional) nanostructure, may comprise a nanorod. The term “nanorod” may herein refer to a nanoscale object having a rodlike shape. The term “nanorod” may herein also refer to a plurality of nanorods. The rodlike shape may be an elongated shape. Further, in embodiments, the nanorod may have a cross-sectional shape perpendicular to the axis of elongation, wherein the cross- sectional shape is selected from the group comprising a round shape, such as a circular shape, a rectangle, such as a square, and a (regular) polygon, such as e.g. a (regular) hexagon.

Specifically, the nanorod may have a length (L) and an equivalent circular diameter (D), wherein both dimensions may be individually selected from the range of 1-500 nm. In embodiments, a nanorod may have an aspect ratio L/D > 2, such as L/D > 3, especially L/D > 5. Further, in embodiments, the nanorod may have an aspect ratio L/D > 10. Hence, in embodiments, a nanorod may have an elongated structure.

In further embodiments, the nanorod may have an aspect ratio L/D < 100, such as < 75, especially < 50.

In embodiments, the equivalent circular diameter (or ECD) (or “circular equivalent diameter”) of an (irregularly shaped) two-dimensional shape may be the diameter of a circle of equivalent area. For instance, the equivalent circular diameter of a square with side a may be 2*a/SQRT(xn). For a circle, the diameter may be the same as the equivalent circular diameter. Would a circle in an xy-plane with a diameter D be distorted to any other shape (in the xy-plane), without changing the area size, then the equivalent circular diameter of that shape may be D.

In embodiments, a nanorod may have a relatively large surface area to volume ratio. This may increase the contact area between the nanorod and the reduction gas and/or oxidation gas, compared to micro- and macroscale materials. Hence, a nanorod may have improved hydrogen production and increased reactivity during the reduction stage and/or the oxidation stage.

In embodiments, the redox material may comprise a plurality of nanorods, such as at least two nanorods. In embodiments, the redox material may comprise at least 50 wt% nanorods, such as at least 70 wt% nanorods, especially at least 90 wt% nanorods, including 100 wt% nanorods. In specific embodiments, the redox material may essentially consist of a plurality of nanorods.

The invention may herein for explanatory purposes primarily be discussed in the context of (cerium dioxide) nanorods. However, it will be clear to the person skilled in the art that the nanostructure may, alternatively or additionally, comprise a nanowire. The terms “nanorod” and “nanowire” may in the art both be used to refer to so-called “one-dimensional” nanostructures, wherein the term “nanorod” may typically be used for nanostructures having a (equivalent circular) diameter and a length in the nanoscale, with an aspect ratio L/D < 100, such as < 50, whereas the term “nanowire” may typically be used for nanostructures having an (equivalent circular) diameter in the nanoscale, but optionally with a length exceeding into the microscale or beyond, i.e, the term “nanowire” may typically be used for nanostructures having an aspect ratio L/D > 100, such as > 1000.

In further embodiments, the redox material, especially the (one-dimensional) nanostructure, may comprise a nanowire. The term “nanowire” may herein refer to a nanoscale object having a wirelike shape. The term “nanowire” may herein also refer to a plurality of nanowires. The wirelike shape may be elongated. Further, in embodiments, the nanowire may have a cross-sectional shape perpendicular to the axis of elongation, wherein the cross-sectional shape is selected from the group comprising a round shape, such as a circular shape, a rectangle, such as a square, and a (regular) polygon, such as e.g. a (regular) hexagon.

Specifically, the nanowire may have a length (L) and an equivalent circular diameter (D), wherein the equivalent circular diameter (D) may be selected from the range of 1-50 nm, and wherein the length (L) may be selected from the range of > 100 nm, such as > 500 nm, especially > 1000 nm. In embodiments, the nanowire may have an aspect ratio L/D > 100, such as L/D > 150, especially L/D > 500. Further, in embodiments, the nanowire may have an aspect ratio L/D > 1000. Hence, in embodiments, a nanowire may have an elongated structure.

As described above, the nanostructure, especially the nanorod, or especially the nanowire, may have a cross-sectional shape (perpendicular to an axis of elongation), wherein the cross-sectional shape is selected from the group comprising a round shape, such as a circular shape, a rectangle, such as a square, and a (regular) polygon, such as e.g. a (regular) hexagon.

In further embodiments, the cross-sectional shape of the nanostructure, especially the of nanorod, or especially of the nanowire, approximates a circular cross-sectional shape. In further embodiments, the cross-sectional shape of the nanostructure, especially the of nanorod, or especially of the nanowire, approximates a rectangular cross-sectional shape. In further embodiments, the cross-sectional shape of the nanostructure, especially the of nanorod, or especially of the nanowire, approximates a polygonal cross-sectional shape, especially a cross- sectional shape approximating a regular polygon.

The term “approximate” and its conjugations herein, such as in “to approximate a shape”, refers to being nearly identical to, especially identical to, the following term, for example nearly identical to a circle or a rectangle. For example, a nanostructure may have a circular cross-sectional shape but for a defect. Similarly, for example, the nanostructure may not have a perfectly round cross-sectional shape but may, for instance, be slightly ellipsoidal.

In particular, an object approximating a first shape may herein refer to: a first shape realization encompassing the object, wherein the first shape realization is defined as the smallest encompassing shape of the (2D or 3D, respectively) object wherein the first shape realization has the shape of the first shape, wherein a ratio of the area (volume) of the first shape realization to the area (volume) of the object is < 1.2, especially < 1.1, such as <1.05, especially <1.02. For instance, a cross-section of the nanorod may approximate a circular shape, wherein the first shape realization may be defined as the smallest encompassing circular shape of the cross- sectional shape, wherein a ratio of the volume of the first shape realization to the volume of the cross-sectional shape 1s < 1.2, especially, especially < 1.1, such as <1.05, especially <1.02, including 1. Further, if the dimensions of the first shape are defined, the term approximate may refer to the object and the first shape being superimposable (in 2D or 3D, respectively) such that an intersection between the object and the first shape covers at least n% of the object and at least n% of the shape, wherein n is at least 90%, such as at least 95%, especially at least 98%, such as at least 99%, including 100%.

In embodiments the method may comprise the reduction stage. The reduction stage may comprise exposing the redox material to a reduction temperature Tred and a reduction gas. In embodiments, exposing the redox material to such conditions may reduce the redox material according to the following reduction reaction: MO, > MOy.; + 4802, wherein M indicates a metal atom, and MO, indicates the reduced redox material. Hence, in embodiments, the reduction stage may comprise providing oxygen gas. In embodiments, the reduction reaction may be endothermic. Specifically, the reduction reaction may be facilitated by elevated temperatures. Hence, in embodiments, the reduction temperature Tres may be selected from the range of > 200 °C, such as from the range of > 250 °C, especially from the range of > 300 °C. Further, in embodiments, Tres may be selected from the range of > 350 °C.

This may (still) be a relatively low temperature for thermochemical water splitting. Hence, such embodiments may facilitate providing (oxygen and) hydrogen gas at a relatively low energy cost. As such, in such embodiments, the energy efficiency of the method may be relatively high.

Additionally, the method, especially the reduction stage, may comprise exposing the redox material to a reduction gas. In embodiments, the reduction gas may comprise an inert gas. An inert gas may be a gas that (essentially) does not react with the reactor material under the conditions provided in the reactor. Hence, in embodiments, the reduction gas may have as a function to remove the oxygen provided by the redox material. In embodiments, the inert gas may comprise one or more of nitrogen gas and a noble gas, such as an argon gas. Further, in embodiments, the reduction gas may comprise at most 2 wt% oxygen, such as at most 1 wt% oxygen, especially at most 0.1 wt% oxygen, including (essentially) no oxygen. Similarly, the reduction gas may comprise at most 2 wt% water vapor, such as at most 1 wt% water vapor, especially at most 0.1 wt% water vapor, including (essentially) no water vapor. Thus, in embodiments, the reduction gas may especially be (essentially) free from oxygen and/or water vapor. As such, in embodiments, the reduction gas may also be referred to as an “oxygen-free gas” or a “water-free gas”. Suitable choices for the inert gas will be apparent to the person skilled in the art.

Furthermore, the method may comprise the oxidation stage. The oxidation stage may comprise exposing the (reduced) redox material to an oxidation gas. In embodiments, the oxidation gas may comprise water vapor. In embodiments, the oxidation gas may comprise at least 5 wt% water vapor, such as at least 20 wt% water vapor, especially at least 50 wt% water vapor. Further, in embodiments, the oxidation gas may comprise (mainly) water vapor, such as at least 60 wt% water vapor, especially at least 75 wt% water vapor, more especially at least 90 wt% water vapor, including 100 wt% water vapor. In certain embodiments, the oxidation gas may (essentially) consist of water vapor. Additionally or alternatively, the oxidation gas may comprise the water vapor, wherein the water vapor may have a water vapor partial pressure

Pw. In embodiments, Py may be selected from the range of 0.1-4 bar, such as from the range of 0.5-3 bar, especially from the range of 0.5-2 bar. In further embodiments, the water vapor may have a water vapor partial pressure Pw > 1 bar, such as > 1.5 bar, especially > 2 bar. Exposing the (reduced) redox material to water vapor may oxidize the (reduced) redox material to regenerate the redox material according to the following oxidation reaction: MOx.s + 6H20 >

MO, + ôH:. Hence, in embodiments, the oxidation stage may provide hydrogen gas.

In embodiments, the method, especially the oxidation stage, may comprise providing hydrogen gas. Specifically, in embodiments, the method, especially the oxidation stage, may comprise providing a product gas (“mixture”) comprising the oxidation gas and the hydrogen gas. In further embodiments, the method may comprise separating the hydrogen gas from (remaining components of) the product gas. It will be clear to the person skilled in the art how a hydrogen gas can be separated from other components of a gaseous mixture. For instance, in embodiments, the method may comprise separating the hydrogen gas from (remaining components of) the product gas using one or more of condensation (especially of the water vapor), pressure swing adsorption, and membrane separation.

The term “stage” and similar terms used herein may refer to a (time) period (also “phase”) of a method and/or an operational mode. It will be clear to the person skilled in the art how the stages may be beneficially arranged in time. In particular, in embodiments, the reduction stage and the oxidation stage may be temporally separated.

The invention may herein for explanatory purposes primarily be described in the context of providing (only) hydrogen gas and, temporally separated therefrom, oxygen gas.

However, the invention may further apply to, for example, the generation of a gaseous mixture, especially a gaseous mixture comprising hydrogen gas, such as a gaseous mixture comprising syngas.

For instance, in embodiments, the oxidation gas may further comprise carbon dioxide (gas). In embodiments, the oxidation gas may comprise at least 5 wt% carbon dioxide, such as at least 20 wt% carbon dioxide, especially at least 50 wt% carbon dioxide. Further, in embodiments, the oxidation gas may comprise at least 5 wt% water vapor, such as at least 20 wt% water vapor, especially at least 50 wt% water vapor. In embodiments, the oxidation gas may comprise equal amounts of carbon dioxide and water vapor. Alternatively, the oxidation gas may comprise different amounts of carbon dioxide and water vapor, especially more carbon dioxide, or especially more water vapor. In specific embodiments, the oxidation gas may (essentially) consist of the carbon dioxide and water vapor. Additionally or alternatively, the oxidation gas may comprise the carbon dioxide, wherein the carbon dioxide may have a carbon dioxide partial pressure Pco:. In embodiments, Pco2 may be selected from the range of 0. 1-4 bar, such as from the range of 0.5-3 bar, especially from the range of 0.5-2 bar. Exposing the (reduced) redox material to carbon dioxide may oxidize the (reduced) redox material to regenerate the redox material according to the following (second) oxidation reaction: MO,.3 +

SCO: 2 MO, + SCO. At the same time, exposing the (reduced) redox material to water vapor may oxidize the (reduced) redox material to regenerate the redox material according to the following oxidation reaction: MO. + ôH:0 > MO, + 3H:. Hence, in embodiments, the oxidation stage may provide a combination of carbon monoxide and hydrogen gas, i.e. syngas.

Further, in embodiments, the oxidation gas may comprise one or more of nitrogen gas, and a noble gas, such as argon gas. In embodiments, the oxidation gas may comprise at least 50 wt%, such as at least 80 wt%, especially at least 95 wt% of the one or more of nitrogen gas and a noble gas. Further, in embodiments, the oxidation gas may comprise at most 50 wt%, such as at most 20 wt%, especially at most 5 wt% of the one or more of nitrogen gas and a noble gas. In certain embodiments, the oxidation gas may be (essentially) free of nitrogen gas and/or a noble gas, especially (essentially) free of (both) the nitrogen gas and the noble gas.

In embodiments, the method, especially the oxidation stage, may comprise controlling the concentration of water vapor in the oxidation gas. Specifically, in embodiments, the oxidation stage may comprise adjusting the concentration of water vapor in the oxidation gas. Thus, in embodiments, the method, especially the oxidation stage, may comprise reducing the concentration of water vapor in the oxidation gas over time, especially during the last 20% of the oxidation stage, such as during the last 10%. This may have as advantage that the concentration of water vapor may be reduced towards the end of the oxidation stage, such that essentially all water vapor may be removed prior to starting the (next) reduction stage.

Hence, in embodiments, the oxidation stage precedes the reduction stage.

Further, in embodiments, the oxidation stage follows the reduction stage. Therefore, in embodiments, the method may comprise alternating between the reduction stage and the oxidation stage.

Furthermore, in embodiments, the oxidation stage may have an oxidation duration (fox). The oxidation duration (fox) may be selected from the range of 5-150 min, such as from the range of 10-120 min, especially from the range of 10-90 min. Analogously, the reduction stage may have a reduction duration (freq). The reduction duration (freq) may be selected from the range of 5-150 min, such as from the range of 10-120 min, especially from the range of 10-90 min. In embodiments, the oxidation stage and the reduction stage may have an equal duration. However, in further embodiments, the oxidation stage and the reduction stage may have a different duration. In embodiments, fRea/tox may be selected from the range of 0.5-2, such as from the range of 0.75-1.25, especially from the range of 0.9-1.1. Thus, in specific embodiments, the reduction stage may have a reduction duration (7req) selected from the range of 10-90 min, and the oxidation stage may have an oxidation duration (fox) (independently) selected from the range of 10-90 min. Hence, in embodiments, the reduction duration (fred) may be larger than the oxidation duration (fox), such that freq > tox, especially wherein fred > 1.1%f0x, such as fred > 1.2*f0, especially fred > 1.5% fox.

In embodiments, the reduction duration (freq) may be selected such that, after the reduction stage, on average, unit cells of the redox material, especially of the nanostructure, more especially of the nanorod, may have at least three oxygen vacancies, such as at least 4 oxygen vacancies, especially at least 5 oxygen vacancies.

The term “unit cell” may herein refer to the smallest building block of a crystal, and may be a representative unit of the repetitive motifs in the crystal structure. Hence, in embodiments, the redox material, especially the nanostructure, more especially the nanorod, may be at least partly crystalline.

In further embodiments, the reduction duration (fred) may be selected such that, on average, at least 3 oxygen vacancies may be generated per unit cell of the redox material, such as at least 4 oxygen vacancies, especially at least 5 oxygen vacancies. Specifically, in embodiments, the reduction duration (red) may be selected such that, on average, at least 3 oxygen atoms may be extracted per unit cell of the redox material at the end of the reduction stage.

Similarly, in embodiments, the oxidation duration (fox) may be selected such that, after the oxidation stage, on average unit cells of the redox material, especially of the nanostructure, more especially of the nanorod, may have at most 2 oxygen vacancies, such as at most 1 oxygen vacancy, especially at most 0.5 oxygen vacancies, including 0 oxygen vacancies.

In further embodiments, the oxidation duration (fox) may be selected such that, on average, at least 3 oxygen vacancies may be filled per unit cell of the redox material, such as at least 4 oxygen vacancies, especially at least 5 oxygen vacancies. Specifically, in embodiments, the oxidation duration (fox) may be selected such that, on average, at least 3 oxygen atoms may be inserted per unit cell of the redox material at the end of the oxidation stage.

In embodiments, the oxidation stage may comprise exposing the redox material to an oxidation temperature Tox. Further, in embodiments, the oxidation temperature Tox may be selected from the range of > 100 °C, such as from the range of > 150 °C, especially from the range of > 200 °C. In embodiments, Tox may be selected from the range of > 250 °C. Heating the redox material to such temperatures may increase the amount of hydrogen provided by the oxidation stage. However, in embodiments, the oxidation reaction may be exothermic, while the reduction reaction may be endothermic. Hence, the oxidation reaction may be facilitated by lower temperatures than the reduction reaction. As such, in embodiments, the oxidation temperature Tox may not be higher than the reduction temperature Trea. Therefore, in embodiments, the oxidation temperature Tox may be equal to the reduction temperature Tred, such that Tox = Tred. In further embodiments, the oxidation temperature Tox may be lower than the reduction temperature Tred, such that Tox < Trea. Hence, in specific embodiments, the method may comprise exposing the redox material to an oxidation temperature Toy in the oxidation stage, wherein the oxidation temperature Tox may be selected from the range of > 200 °C, and wherein Tox < Tred. In further embodiments, the oxidation temperature Tox may be equal to or higher than the reduction temperature Tred, such that Tox > Tred. Selecting the oxidation temperature Tox and the reduction temperature Trea to be (essentially) equal may reduce the duration of the reduction stage and the oxidation stage, as the reactor material may not have to be heated and/or cooled down between the reduction stage and the oxidation stage.

On the other hand, selecting the oxidation temperature Tox to be lower than the reduction temperature Tres may improve the reaction conditions for both the reduction reaction and the oxidation reaction, thereby increasing the amount of hydrogen gas provided by the method.

Thus, in embodiments, Trea — Tox > 50°C, such as Tred — Tox > 60°C, especially Tred — Tox > 70°C. For example, in embodiments, Tred may be selected from the range of > 400 °C, Tox may be selected from the range of > 300°C, and Trea — Tox > 100°C. However, in specific embodiments, Tred — Tox < 50°C, such as < 25 °C, especially < 10°,

In embodiments, the reduction temperature Trea may further be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C. Analogously, in embodiments, the oxidation temperature Tox may be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C. Hence, in specific embodiments, reduction temperature Trea may be selected from the range of < 750 °C, and the oxidation temperature Tox may be selected from the range of < 750 °C. Further, in embodiments, the reduction temperature Tres may be selected from the range of < 650 °C, such as < 550 °C, especially < 450 °C. Analogously, in embodiments, the oxidation temperature Tox may be selected from the range of < 650 °C, such as < 550 °C, especially < 450 °C.

In further embodiments, the reduction temperature Tres may be selected from the range of 300 — 500 °C, especially from the range of 350 — 450 °C, and Tox may be selected from the range of 200 — 400 °C, especially from the range of 250 — 350 °C, and especially wherein Trea-Tox > 10 °C, such as > 25 °C.

An advantage of such a relatively low reduction temperature Tres and oxidation temperature Tox may be that (waste) heat from e.g. existing chemical reactions, production facilities, power plants, processing facilities, or nuclear (power) plants may be used to heat the reactor material. Hence, in such embodiments, relatively little, especially (essentially) no, (additional) energy may be required for heating of the reactor material, reducing the energy cost of the method, 1.e., such embodiments may be particularly energy-efficient.

Furthermore, as indicated above, the reactor material may comprise a redox material. Hence, in embodiments, the reduction gas may be supplied to the redox material.

Further still, the reduction gas may have a pressure Pred. So, in embodiments, the reduction gas may be supplied to the redox material at a pressure Pred. In embodiments, the reduction gas may have a pressure Pre selected from the range of 0. 1-2 bar, such as 0.3-1 bar, especially 0.5- 1 bar. Similarly, the oxidation gas may have a pressure Pox. In embodiments, the oxidation gas may be supplied to the reactor at a pressure Pox. Further, in embodiments, the oxidation gas may have a pressure Poy selected from the range of 1-20 bar, such as from the range of 1-10 bar, especially from the range of 1-5 bar. It may, in embodiments, be advantageous to supply the reduction gas and the oxidation gas at different pressures Pisa and Po, respectively. For example, in the reduction stage, oxygen may be extracted from the redox material. This may be better facilitated by a low pressure Pred, which may help the oxygen gas escape the redox material and the reactor. In contrast, a higher pressure Pox may facilitate the water splitting reaction in the oxidation stage. In embodiments, a higher pressure Pox may increase contact between the water vapor and the redox material. However, a high pressure Pox may also trap hydrogen gas within the redox material. This may slow down the water splitting reaction, thereby decreasing the amount of hydrogen provided in the oxidation stage. Thus, in embodiments, Pox - Pred < 5 bar, especially Pox - Pred < 1 bar, such as Pox - Pred < 0.5 bar, especially Pox - Prea < 0.25 bar. Further, in embodiments, Pox - Prea > 1 bar, such as Pox - Pred > 3 bar, especially Pox - Pred > 5 bar. Additionally or alternatively, in embodiments, O bar < Poy - Pred < 60 bar, such as 0 bar < Pox - Prea < 45 bar, especially 1 bar < Poy - Preq < 45 bar, more especially 1 bar < Pox - Prea < 30 bar. Hence, in specific embodiments, the method may comprise supplying the reduction gas to the redox material at a pressure Pre, and supplying the oxidation gas to the reactor at a pressure Pox, wherein 1 bar < Poy - Pred < 30 bar. Further, in specific embodiments, Pox - Pred > 1 bar.

As indicated above, in embodiments, the method may comprise providing hydrogen using a reactor material. Here below, features related to the reactor material are described.

In embodiments, the reactor material may comprise a support material. In such embodiments, the support material may be configured to support the redox material. Hence, in embodiments, the redox material may be configured on top of the support material. In alternative embodiments, the redox material may be configured within the support material.

Specifically, in embodiments, the redox material may be dispersed throughout the support material. As such, the support material may provide increased access to the redox material for the water vapor. This may especially be the case when a porous support material is selected.

Further, in embodiments, the support material may spatially separate the nanostructures, especially the nanorods, or especially the nanowires, comprised by the redox material. This may reduce sintering of the redox material upon exposure to heat during the reduction stage and/or oxidation stage. Hence, in embodiments the support material may comprise a thermally stable material. Additionally or alternatively, in embodiments, the support material may comprise a non-reactive material. In embodiments, the support material may be selected from the group comprising alumina, silica, titania, and zirconia. However, other suitable materials not indicated herein may also be selected. Such materials will be known to a person skilled in the art. In embodiments, the reactor material may comprise at least 40 wt% support material, such as at least 55 wt% support material, especially at least 70 wt% support material.

Furthermore, in embodiments, the reactor material may comprise at least 80 wt% support material, such as at least 90 wt% support material. In further embodiments, the reactor material may comprise at most 99 wt% support material, such as at most 90 wt% support material, especially at most 80 wt% support material. In further embodiments, the reactor material may comprise at most 70% support material, such as at most 60 wt% support material, especially at most 50 wt% support material.

Additionally, in embodiments the reactor material may comprise iron oxide. In embodiments, the reactor material may comprise at most 70 wt% iron oxide, such as at most wt% iron oxide. In further embodiments, the reactor material may comprise at most 35 wt% iron oxide. Hence, in specific embodiments, the reactor material may comprise a support material, wherein the support material may be configured to support the redox material,

wherein the support material may comprise a thermally stable and non-reactive material, and wherein the reactor material may comprise at most 35 wt% iron oxide.

Further, in embodiments the reactor material may comprise the redox material, wherein the redox material may comprise a (one-dimensional) nanostructure, such as a nanorod. As indicated above, in embodiments, the nanorod may have a length (L) and an equivalent circular diameter (D), wherein both dimensions may be individually selected from the range of 1-500 nm. In embodiment, the length (L) may further be selected from the range of 5-400 nm, such as 5-300 nm, especially 10-200 nm. In embodiments, the length (L) may be selected from the range of 20-100 nm. Additionally, in embodiments, the equivalent circular diameter (D) may be selected from the range of 1-100 nm, such as 1-50 nm, especially 1-20 nm. In further embodiments, the equivalent circular diameter (D) may be selected from the range of 2-10 nm. Hence, in specific embodiments, the nanorod may have a length (L) selected from the range of 10-200 nm and an equivalent circular diameter (D) selected from the range of 1-20 nm. Further, in embodiments, the nanorod may have a deviation in the equivalent circular diameter (D) of < 15%, such as < 10%, especially < 5% over at least 60%, such as at least 70%, especially at least 80% of the length (L) of the nanorod.

Alternatively or additionally, in embodiments the redox material may comprise a (one-dimensional) nanostructure, such as a nanowire. In embodiment, the length (L) of the nanowire may be selected from the range of 100-2000 nm, such as 100-1500 nm, especially 100-1000 nm. In embodiments, the length (L) may be selected from the range of 100-500 nm.

Additionally, in embodiments, the equivalent circular diameter (D) may be selected from the range of 1-100 nm, such as 1-50 nm, especially 1-20 nm. In further embodiments, the equivalent circular diameter (D) may be selected from the range of 2-10 nm. Hence, 1n specific embodiments, the nanowire may have a length (L) selected from the range of 100-1000 nm and an equivalent circular diameter (D) selected from the range of 1-20 nm. Further, in embodiments, the nanowire may have a deviation in the equivalent circular diameter (D) of < 15%, such as < 10%, especially < 5% over at least 60%, such as at least 70%, especially at least 80%, of the length (L) of the nanowire.

In embodiments, the nanostructure, especially the nanorod, or especially the nanowire, may comprise a crystalline material. In other embodiments, the nanostructure (itself) may be (essentially) crystalline. Further, in embodiments, the nanostructure, especially the nanorod, or especially the nanowire, may have one or more sides. The one or more sides may, in embodiments, comprise a crystal plane, selected from the group comprising {001}, {100}, {110}, and {111} crystal planes, as denoted in Miller indices. In embodiments, the nanostructure shape, especially the nanorod shape, or especially the nanowire shape, may allow at least one of the one or more sides to preferentially comprise {110} and {100} crystal planes.

Hence, in specific embodiments, the nanostructure may be crystalline, wherein the nanostructure may comprise one or more sides, and wherein at least one of the one or more sides of the nanostructure may comprise {110} and {100} crystal planes. Further, in embodiments, at least 35%, such as at least 50%, especially at least 60% of the one or more sides may comprise {110} and {100} crystal planes. In embodiments, the {110} and {100} crystal planes may be more reactive than the {001} and {111} crystal planes. This may, in embodiments, allow more oxygen to be extracted from the nanostructure in the reduction stage, allowing for an increased hydrogen production in the oxidation stage.

Further, in embodiments, the redox material may comprise a plurality of nanostructures, such as a plurality of nanorods, such as at least two nanorods. In embodiments, two or more nanorods of the plurality of nanorods may be (reversibly) physically connected.

For example, in embodiments, at least one nanorod of the two or more nanorods may be adsorbed onto (a side of) another of the two or more nanorods. Hence, in embodiments, two or more nanorods of the plurality of nanorods may form a nanorod bundle. In embodiments, the two or more nanorods comprised by the nanorod bundle may be (reversibly) physically connected with each other. Hence, in specific embodiments, the redox material may comprise a nanorod bundle, wherein the nanorod bundle may comprise at least two nanorods, wherein the at least two nanorods may be (reversibly) physically connected. Furthermore, in embodiments, the nanorod bundle may comprise at least 5 nanorods, such as at least 10 nanorods. Further still, in embodiments, the nanorod bundle may comprise at most 30 nanorods, such as at most 20 nanorods, especially at most 15 nanorods. In embodiments, the nanorods within the nanorod bundle may be oriented (essentially) in parallel. Yet, in alternative embodiments, the nanorods within the nanorod bundle may be oriented in different directions.

In embodiments, a nanorod bundle may be more thermally stable than an (isolated) nanorod.

Hence, a redox material comprising a nanorod bundle may have a longer operational lifetime.

Therefore, in embodiments, the redox material may comprise a plurality of nanorod bundles, such as at least two nanorod bundles.

Yet, in embodiments, the redox material may comprise a plurality of single (physically separated) nanostructures, such as single (physically separated) nanorods, 1.e., nanorods not being part of a nanorod bundle. In particular, in embodiments, at least 50% of the nanorods may be single nanorods, such as at least 70%, especially at least 80%, such as at least 90%, including (essentially) 100%.

According to a second aspect, the invention provides a system for providing hydrogen. Specifically, in embodiments, the system may provide hydrogen using the method as described herein. In embodiments, the system may comprise a reactor. Further, in embodiments, the system may comprise a temperature control unit. Further still, in embodiments the system may comprise a gas supply. In embodiments, the reactor may be configured to host a reactor material. The reactor material may comprise a redox material. In such embodiments, the redox material may comprise a (one-dimensional) nanostructure selected from the group comprising a nanorod and a nanowire. Additionally, in embodiments, the redox material may comprise > 25 wt% cerium dioxide, such as > 40 wt% cerium dioxide, especially > 50 wt% cerium dioxide. Further, in embodiments, the redox material may comprise 0-50 wt% iron oxide, such as 1-40 wt% iron oxide, especially 1-25 wt% iron oxide. In embodiments, the system may be configured to switch between a reduction stage and an oxidation stage. Specifically, in embodiments, in the reduction stage, the gas supply may be configured to provide a reduction gas to the reactor. In embodiments, the reduction gas may comprise an inert gas. Furthermore, in embodiments, in the reduction stage, the temperature control unit may be configured to control a reduction temperature Trea of the reactor. In such embodiments, the reduction temperature Trea may be selected from the range of > 200 °C, such as > 250 °C, especially > 300 °C. Meanwhile, in embodiments, in the oxidation stage, the gas supply may be configured to provide an oxidation gas to the reactor. In such embodiments, the oxidation gas may comprise water vapor.

Hence, in specific embodiments, the invention provides a system for providing hydrogen, wherein the system comprises a reactor, a temperature control unit, and a gas supply, wherein the reactor is configured to host a reactor material comprising a redox material, wherein the redox material comprises a nanostructure selected from the group comprising a nanorod and a nanowire, and wherein the redox material comprises > 50 wt% cerium dioxide, and wherein the redox material comprises 1-25 wt% iron oxide, wherein the system is configured to switch between a reduction stage and an oxidation stage, wherein: 1) in the reduction stage the gas supply is configured to provide a reduction gas to the reactor, wherein the reduction gas comprises an inert gas, and wherein in the reduction stage the temperature control unit is configured to control a reduction temperature Trea of the reactor, wherein the reduction temperature Treq is selected from the range of > 300 °C; ii) in the oxidation stage the gas supply is configured to provide an oxidation gas to the reactor, wherein the oxidation gas comprises water vapor.

Such a system may facilitate generating hydrogen at a relatively low energy cost (for a thermochemical water splitting process). Furthermore, such a system may allow hydrogen production at lower reduction temperatures Tred. Additionally, with such a system, the lifetime of the redox material and/or the (one-dimensional) nanostructure, especially the nanorod, or especially the nanowire, may be extended due to the lower reduction temperature

Tred. Additionally, a redox material and/or a nanostructure with > 50 wt% cerium dioxide may be thermally stable. As such, the gas stream provided during the reduction stage and/or oxidation stage may not need to be gas quenched. In embodiments, a redox material and/or a nanostructure, especially a nanorod, further comprising 1-25 wt% iron oxide may show an increased hydrogen production rate compared to redox materials and/or nanostructures comprising 0 wt% iron oxide, or >25 wt% iron oxide. Hence, in embodiments, the method may provide a relatively high hydrogen production at a relatively low temperature. This may facilitate generating hydrogen at a relatively low energy cost (for a thermochemical water splitting process). Additionally, the redox material comprising a nanostructure, especially a nanorod, or especially a nanowire, may be reduced relatively quickly, i.e., the redox material may facilitate having a relatively short reduction time, which may facilitate increasing cycling between the reduction stage and the oxidation stage, and thus increasing the overall hydrogen production rate.

In embodiments, the system may comprise the reactor. As indicated above, a reactor may be a vessel in which chemical reactions may occur, such as a batch reactor, a continuous stirred-tank reactor (CSTR), a plug flow reactor (PFR), a semibatch reactor, a catalytic reactor, or a nuclear reactor. Hence, in embodiments, the reactor may comprise a reaction chamber. The reaction chamber may be a compartment in which the chemical reaction is performed. Further, the reactor may comprise a reactor inlet and a reactor outlet. In embodiments, the reactor inlet and the reactor outlet may be fluidically connected to the reaction chamber. Additionally, the reactor may, in embodiments, be a thermochemical reactor, such as a furnace reactor, a solar reactor, an infrared reactor, a microwave reactor, a fixed bed reactor, a packed bed reactor, a fluidized bed reactor, a moving bed reactor, a membrane reactor, a plasma reactor, or a flow reactor. Hence, in embodiments, the system, especially the reactor, may comprise a heating element. The heating element may be configured to heat the reaction chamber. Hence, in embodiments, the heating element may at least partially enclose the reaction chamber.

Further, in embodiments, the system may comprise a gas supply. The gas supply may comprise one or more gas supply outlets. The gas supply may be configured to supply the reduction gas to the reactor. Further, in embodiments, the gas supply may be configured to supply the oxidation gas to the reactor. Specifically, in embodiments, the gas supply may supply the reduction gas and/or the oxidation gas to the reactor inlet. In embodiments, the gas supply may be configured to supply the reduction gas to the reactor during the reduction stage.

Further, the gas supply may be configured to supply the oxidation gas to the reactor during the oxidation stage. Hence, in embodiments, the gas supply may be configured to supply the reduction gas and the oxidation gas to the reactor at different times. Specifically, at any time during operation of the system, the gas supply may, in embodiments, be configured to supply the reduction gas or the oxidation gas to the reactor. Further, in embodiments, the gas supply may be configured to supply the reduction gas and the oxidation gas to the reactor through one of the one or more gas supply outlets and another of the one of more gas supply outlets, respectively. In embodiments, the gas supply may be fluidically connected to the reactor inlet.

In embodiments, the gas supply may comprise one or more mass flow controllers. Thus, in embodiments, the gas supply may be configured to control the flow rate of one or more of the reduction gas and the oxidation gas. Further, in embodiments, the gas supply may be configured to heat the reduction gas and/or the oxidation gas. Specifically, the gas supply may heat the reduction gas and/or the oxidation gas in the gas supply. Hence, in embodiments, the gas supply may be configured to supply a heated gas stream, wherein the gas stream may comprise one or more of the reduction gas and the oxidation gas. Analogously, the gas supply may be configured to pressurize the reduction gas and/or the oxidation gas. Hence, in embodiments, the gas supply may comprise a pressure control unit. In such embodiments, the pressure control unit may be configured to adjust and/or maintain the pressure of the reduction gas and/or the oxidation gas.

Specifically, the gas supply may pressurize the reduction gas and/or the oxidation gas in the gas supply. Hence, in embodiments, the gas supply may be configured to supply a pressurized gas stream, wherein the gas stream may comprise the reduction gas or the oxidation gas, especially the reduction gas, or especially the oxidation gas. Additionally or alternatively, the gas supply may also be configured to supply a gas stream with reduced pressure (i.e, a gas stream with a pressure below 1 bar), wherein the gas stream may comprise the reduction gas or the oxidation gas, especially the reduction gas, or especially the oxidation gas.

Additionally, in embodiments, the system may comprise a temperature control unit. In embodiments, the temperature control unit may be configured to a) measure the temperature in the reactor, and/or to b) control the heating element to adjust the temperature to a previously set temperature. In embodiments, the temperature control unit may be configured to measure the temperature close to the reaction chamber, such as in the reaction chamber.

Hence, in embodiments, the temperature control unit may comprise a temperature sensor. In embodiments, the temperature sensor may be a contact type temperature sensor. Alternatively, the temperature sensor may be a non-contact type temperature sensor. In embodiments, the temperature sensor may further be selected from the group comprising a negative temperature coefficient thermistor, a positive temperature coefficient thermistor, a resistive temperature detector, a thermocouple, a semiconductor-based sensor, and a thermostat. Additionally, in embodiments, the temperature control unit may comprise a driver unit. The driver unit may be configured to control the heating element. Especially, in embodiments, the driver unit may be configured to control the heating element based on a signal from the temperature sensor.

However, in embodiments, the driver unit may (also) be configured to control the heating element based on an external signal. In embodiments, the external signal may be provided by a control system.

In embodiments, the temperature control unit may be configured to, in the reduction stage, control the reduction temperature Treq of the reactor. Additionally, the temperature control unit may be configured to, in the oxidation stage, control an oxidation temperature Tox of the reactor. In embodiments, the oxidation temperature Tox may be selected from the range of > 100 °C, such as > 150 °C, especially > 200 °C. Further, in embodiments, the oxidation temperature Tox may be equal to or higher than the reduction temperature Tred, such that Tox > Tred. Alternatively, in embodiments, the oxidation temperature Tox may be equal to or lower than the reduction temperature Tred, such that Tox < Tred. Hence, in specific embodiments, the temperature control unit may be configured to control an oxidation temperature Tox of the reactor in the oxidation stage, wherein the oxidation temperature Tox may be selected from the range of > 200 °C, and Tox < Tred.

In further embodiments, the oxidation temperature Tox and the reduction temperature Trea may differ < 50 °C, such as < 25 °C, especially < 10 °C. In particular, in embodiments, the oxidation temperature Tox may be (essentially) equal to the reduction temperature Treg (i.e. the process may be isothermal), such that Tox = Trea. Such embodiments may be particularly convenient and efficient in terms of thermal regulation.

Further, in embodiments, the reduction temperature Treq may be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C. Analogously, in embodiments, the oxidation temperature Tox may be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C. Hence, in specific embodiments, the invention may provide a system as described herein, wherein the reduction temperature Treg < 750 °C, and wherein the oxidation temperature Tox < 750 °C. Further, in embodiments, the reduction temperature Trea may be selected from the range of < 650 °C, such as < 550 °C, especially < 450 °C. Analogously, in embodiments, the oxidation temperature Tox may be selected from the range of < 650 °C, such as < 550 °C, especially <450 °C.

In embodiments, the heating element and/or the temperature control unit may be configured to heat the reactor using (waste) heat from e.g existing chemical reactions, production facilities, power plants, processing facilities, or nuclear (power) plants. Further, in such embodiments, the gas supply may be configured to heat the reduction gas and/or the oxidation gas using (waste) heat from e.g. existing chemical reactions, production facilities, power plants, processing facilities, or nuclear (power) plants.

As indicated above, the gas supply may be configured to pressurize one or more of the reduction gas and the oxidation gas. Hence, in embodiments, the gas supply may be configured to, in the reduction stage, provide the reduction gas to the reactor at a reduction gas pressure Pred. In embodiments, the reduction gas may have a reduction gas pressure Pred (or “pressure Pred”) selected from the range of 0.1-2 bar, such as 0.3-1 bar, especially 0.5-1 bar.

Furthermore, in embodiments, the gas supply may be configured to, in the oxidation stage, provide the oxidation gas to the reactor at an oxidation gas pressure Pox. In embodiments, the oxidation gas may have an oxidation gas pressure Pox (or “pressure Po") selected from the range of 1-20 bar, such as 1-10 bar, especially 1-5 bar. As indicated above, it may, in embodiments, be advantageous to supply the reduction gas and the oxidation gas at different pressures Pisa and Poy, respectively. Hence, in embodiments, 0 bar < Poy — Pred < 60 bar, such as 0 bar < Pox — Prea < 45 bar, especially 1 bar < Pox — Pred < 45 bar, more especially 1 bar <

Pox — Pred < 30 bar. Thus, in specific embodiments, in the reduction stage the gas supply may be configured to provide the reduction gas to the reactor at a reduction gas pressure Pred, and in the oxidation stage the gas supply may be configured to provide the oxidation gas to the reactor at an oxidation gas pressure Pox, wherein 1 bar < Poy — Prea < 30 bar.

In embodiments, the oxidation gas may comprise water vapor. Additionally, in embodiments, the oxidation gas may further comprise carbon dioxide (gas). In embodiments, the oxidation gas may comprise at least 5 wt% carbon dioxide, such as at least 20 wt% carbon dioxide, especially at least 50 wt% carbon dioxide. Further, in embodiments, the oxidation gas may comprise at least 5 wt% water vapor, such as at least 20 wt% water vapor, especially at least 50 wt% water vapor. Hence, in embodiments, the gas supply may be configured to, in the oxidation stage, provide the oxidation gas to the reactor, wherein the oxidation gas comprises carbon dioxide and water vapor. As such, in embodiments, the system may be a system for providing a combination of carbon monoxide and hydrogen gas, i.e. syngas.

In embodiments described above, the driver unit may be configured to control the heating element based on an external signal, wherein the external signal may be provided by a control system.

Thus, in embodiments, the system may comprise a control system. In embodiments, the control system may (be configured to) control one or more of the reactor, the gas supply, and the temperature control unit. Specifically, the control system may (be configured to) control the flow rate of the reduction gas and/or the oxidation gas. Furthermore, the control system may (be configured to) control the concentration of water vapor in the oxidation gas. Additionally, in embodiments, the control system may (be configured to) control the duration of one or more of the reduction stage and the oxidation stage. Hence, the control system may be configured to, in embodiments, start and/or stop the supply of reduction gas and/or oxidation gas to the reactor. Similarly, the control system may be configured to control the temperature control unit. Specifically, in embodiments, the control system may be configured to control the driver unit. The control system may thus be configured to, in embodiments, control the temperature in the reaction chamber. Therefore, in embodiments, the control system may be configured to control when the reactor may be set to the reduction temperature Treg Similarly, in embodiments, the control system may be configured to control when the reactor may be set to the oxidation temperature Tox. Further, the control system may, in embodiments, be configured to (have the system) execute the method described herein. Thus, in specific embodiments, the system may comprise a control system, wherein the control system may be configured to execute the method of the invention.

The term “controlling” and similar terms herein may especially refer at least to determining the behavior or supervising the running of an element. Hence, herein “controlling” and similar terms may e.g. refer to imposing behavior to the element (determining the behavior or supervising the running of an element), etc., such as e.g. measuring, displaying, actuating, opening, shifting, changing temperature, etc.. Beyond that, the term “controlling” and similar terms may additionally include monitoring. Hence, the term “controlling” and similar terms may include imposing behavior on an element and also imposing behavior on an element and monitoring the element. The controlling of the element can be done with a control system. The control system and the element may thus at least temporarily, or permanently, functionally be coupled. The element may comprise the control system. In embodiments, the control system and the element may not be physically coupled. Control can be done via wired and/or wireless control. The term “control system” may also refer to a plurality of different control systems,

which especially are functionally coupled, and of which e.g. one master control system may be a control system and one or more others may be slave control systems.

In embodiments, the method may comprise providing hydrogen using a reactor material. Thus, in embodiments, the system, especially the reactor, may comprise a reactor material. In embodiments, the reactor material may comprise a redox material. Yet, in embodiments, the reactor material may further comprise a support material. In such embodiments, the support material may be configured to support the redox material. Hence, in embodiments, the redox material may be configured on top of the support material. In alternative embodiments, the redox material may be configured within the support material.

Specifically, in embodiments, the redox material may be dispersed throughout the support material. Additionally, in embodiments, the reactor material may comprise iron oxide. In embodiments, the reactor material may comprise at most 70 wt% iron oxide, such as at most 55 wt% iron oxide. In further embodiments, the reactor material may comprise at most 35 wt% iron oxide. Hence, in specific embodiments, the reactor material may further comprise a support material, wherein the redox material may be dispersed throughout the support material, and wherein the reactor material may comprise at most 35 wt% iron oxide.

The system, especially the control system, may have an operational mode. The term “operational mode” may also be indicated as “controlling mode”. The system, or apparatus, or device (see further also below) may execute an action in a “mode” or “operational mode” or “mode of operation”. Likewise, in a method an action, stage, or step may be executed in a “mode” or “operation mode” or “mode of operation”. This does not exclude that the system, or apparatus, or device may also be adapted for providing another operational mode, or a plurality of other operational modes. Likewise, this does not exclude that before executing the mode and/or after executing the mode one or more other modes may be executed. However, in embodiments, a control system (see further also below) may be available, that is adapted to provide at least the operational mode. Would other modes be available, the choice of such modes may especially be executed via a user interface, though other options, like executing a mode in dependence of a sensor signal or a (time) scheme, may also be possible. The operational mode may in embodiments also refer to a system, or apparatus, or device, that can only operate in a single operational mode (i.e. “on”, without further tunability).

Thus, in an operational mode of the system, the reactor comprised by the system may (be configured to host) the reactor material. Further, the system may, especially in the operational mode, be configured to switch between a reduction stage and an oxidation stage. In particular, the operational mode may comprise the reduction stage and the oxidation stage. In embodiments, in the reduction stage, the gas supply may (be configured to) expose the reactor material to a reduction gas. Additionally or alternatively, in the reduction stage, the temperature control unit may (be configured to) expose the redox material to a reduction temperature Treg.

Furthermore, in embodiments, in the oxidation stage, the gas supply may (be configured to) expose the redox material to an oxidation gas. Further still, in the oxidation stage, the temperature control unit may, in embodiments, (be configured to) expose the redox material to an oxidation temperature Tox. In embodiments, the gas supply may (be configured to), in the reduction stage, expose the redox material to a reduction gas having a pressure Pred. In further embodiments, the gas supply may (be configured to), in the oxidation stage, expose the redox material to an oxidation gas having a pressure Pox.

According to a third aspect, the invention provides a use of a redox material for providing hydrogen from water vapor. In embodiments, the redox material may comprise a (one-dimensional) nanostructure selected from the group comprising a nanorod and a nanowire.

Further, in embodiments, the redox material may comprise > 25 wt% cerium dioxide, such as > 40 wt% cerium dioxide, especially > 50 wt% cerium dioxide. Additionally, in embodiments, the redox material may comprise 0-50 wt% iron oxide, such as 1-40 wt% iron oxide, especially 1-25 wt% iron oxide.

Hence, in specific embodiments, the invention provides a use of a redox material for providing hydrogen from water vapor, wherein the redox material comprises a nanostructure selected from the group comprising a nanorod and a nanowire, wherein the redox material comprises > 50 wt% cerium dioxide, and wherein the redox material comprises 1-25 wt% iron oxide.

Additionally, in embodiments, the redox material as described herein may be used to provide syngas, a mixture of carbon monoxide and hydrogen. In such embodiments, an oxidation gas comprising carbon dioxide and water vapor may be provided to the redox material. Hence, in a further aspect, the invention provides a use of the redox material for providing syngas from carbon dioxide and water vapor. In embodiments, the redox material as described here may replace alternative redox materials in existing and future methods and/or systems for the production of (at least) hydrogen gas.

The embodiments described herein are not limited to a single aspect of the invention. For example, an embodiment describing the method may, for example, further relate to the system, especially to an operational mode of the system, or especially to the control system. Similarly, an embodiment of the system describing an operation of the system may further relate to embodiments of the method. In particular, an embodiment of the method describing an operation (of the system) may indicate that the system may, in embodiments, be configured for and/or be suitable for the operation. Similarly, an embodiment of the system describing actions of (a stage in) an operational mode may indicate that the method may, in embodiments, comprise those actions.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. 1 schematically depicts an embodiment of the invention. Fig. 2A-B schematically depict embodiments of the reactor material 100. Fig. 3 depicts an embodiment of the nanorod 10. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 schematically depicts an embodiment of the system 1000 of the invention which may, in an operational mode, execute the method of the invention. As depicted, the system 1000 may comprise a reactor 200, a temperature control unit 500, and a gas supply 400.

In embodiments, the reactor 200 may be configured to host a reactor material 100 comprising a redox material 110. Further, in embodiments, the redox material 110 may comprise a nanostructure 1 selected from the group comprising a nanorod 10 and a nanowire 20.

Additionally, the redox material 110 may comprise > 50 wt% cerium dioxide. Additionally, the redox material 110 may comprise 1-25 wt% iron oxide. In embodiments, the system 1000 may be configured to switch between a reduction stage and an oxidation stage, especially in an operational mode (of the system). Specifically, in embodiments, in the reduction stage, the gas supply 400 may be configured to provide a reduction gas 410 to the reactor 200. In embodiments, the reduction gas 410 may comprise an inert gas. Furthermore, in embodiments, in the reduction stage, the temperature control unit 500 may be configured to control a reduction temperature Treg of the reactor 200. In such embodiments, the reduction temperature Trea may be selected from the range of > 300 °C. Meanwhile, in embodiments, in the oxidation stage, the gas supply 400 may be configured to provide an oxidation gas 420 to the reactor 200. In such embodiments, the oxidation gas 420 may comprise water vapor.

As depicted, in embodiments, the reactor 200 may host the reactor material 100.

In embodiments, the reactor 200 may host the reactor material 100 in the reaction chamber 230.

As such, in embodiments, the reactor 200 may (at least partially) enclose the reactor material

100. Furthermore, the reactor 200 may comprise a reactor inlet 210 and a reactor outlet 220. In embodiments, the reactor inlet 210 may comprise an inlet valve 211. The inlet valve 211 may be configured to, in an operational mode of the system, allow flowing of the reduction gas 410 and/or the oxidation gas 420 from the gas supply 400 to the reaction chamber 230. Similarly, in embodiments, the reactor outlet 220 may comprise an outlet valve 221. The outlet valve 221 may be configured to, in an operational mode of the system, facilitate removing the oxygen gas 0 and/or the hydrogen gas H; from the reaction chamber 230.

In embodiments, the gas supply 400 may be fluidically connected to the reactor inlet 210. Thus, in embodiments, the gas supply 400 may be fluidically connected to the reaction chamber 230, especially to the reactor material 100.

In embodiments, the temperature control unit 500 may be configured to, in the oxidation stage, control an oxidation temperature Tox of the reactor 200. In embodiments, the oxidation temperature Tox may be selected from the range of > 100 °C, such as > 150 °C, especially > 200 °C. Further, in embodiments, the oxidation temperature Tox may be equal to or higher than the reduction temperature Tred, such that Tox > Trea. Alternatively, in embodiments, the oxidation temperature Tox may especially be equal to or lower than the reduction temperature Tred, Such that Tox < Tred. Further, in embodiments, the temperature control unit 500 may be configured to control the reduction temperature Tree, Wherein the reduction temperature Treq may be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C. Analogously, in embodiments, the oxidation temperature Tox may be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C.

In embodiments, the gas supply 400 may be configured to pressurize one or more of the reduction gas 410 and the oxidation gas 420. Hence, in embodiments, the gas supply 400 may be configured to, in the reduction stage, provide the reduction gas 410 to the reactor 200 at a reduction gas pressure Prea. Furthermore, in embodiments, the gas supply 400 may be configured to, in the oxidation stage, provide the oxidation gas 420 to the reactor 200 at an oxidation gas pressure Pox. In embodiments, the reduction gas 410 and the oxidation gas 420 may have different pressures Preg and Poy, respectively. Specifically, in embodiments, 0 bar < Pox — Pred < 60 bar, such as 0 bar < Pox — Pred < 45 bar, especially 1 bar <Pox— Pred <45 bar, more especially 1 bar < Pox — Prea < 30 bar.

In embodiments, the reactor material 100 may comprise a support material 120.

In such embodiments, the support material 120 may be configured to support the redox material 110. Hence, in embodiments, the redox material 110 may be configured on top of the support material 120. In alternative embodiments, the redox material 110 may be configured within the support material 120. Specifically, in embodiments, the redox material 110 may be dispersed throughout the support material 120 (see also Fig. 2A). Additionally, in embodiments, the reactor material 100 may comprise iron oxide. In embodiments, the reactor material 100 may comprise at most 70 wt% iron oxide, such as at most 55 wt% iron oxide. In further embodiments, the reactor material may comprise at most 35 wt% iron oxide. Further, in embodiments, the redox material 110 may comprise a nanorod bundle 11 (see also Fig. 2A).

In embodiments, the system 1000 may comprise a control system 300. In embodiments, the control system 300 may control one or more of the reactor 200, the gas supply 400, and the temperature control unit 500. Specifically, the control system 300 may control the flow rate of the reduction gas 410 and/or the oxidation gas 420. Furthermore, the control system 300 may control the concentration of water vapor in the oxidation gas 420. Additionally, the control system 300 may be configured to control the temperature control unit 500. The control system 300 may thus be configured to, in embodiments, control the temperature in the reactor 200. As such, the control system 300 may, in embodiments, especially be configured to (have the system 1000) execute an operational mode wherein the system 1000 executes the method of the invention.

In embodiments, the method for providing hydrogen using a reactor material 100 (and hence the operational mode of the system 1000) may comprise a reduction stage and an oxidation stage. Further, in embodiments, the reactor material 100 may comprising the redox material 110. The redox material 110 may further comprise a (one-dimensional) nanostructure 1 selected from the group comprising a nanorod 10 and a nanowire 20. Additionally, the redox material 110 may comprise > 25 wt% cerium dioxide, such as > 40 wt% cerium dioxide, especially > 50 wt% cerium dioxide. In embodiments, the redox material 110 may further comprise 0-50 wt% iron oxide, such as 1-40 wt% iron oxide, especially 1-25 wt% iron oxide, such as 3-20 wt% iron oxide, especially 7-13 wt% iron oxide. In embodiments, the reduction stage may comprise exposing the redox material 110 to a reduction temperature Treq and the reduction gas 410. In embodiments, the reduction temperature Tres may be selected from the range of > 200 °C, such as > 250 °C, especially > 300 °C. Further, in embodiments, the reduction gas 410 may comprise an inert gas. In embodiments, the oxidation stage may comprise exposing the redox material 110 to the oxidation gas 420. In such embodiments, the oxidation gas 420 may comprise water vapor.

In embodiments, the method may comprise exposing the redox material 110 to an oxidation temperature Tox in the oxidation stage. In such embodiments, the oxidation temperature Tox may be selected from the range of > 100 °C, such as > 150 °C, especially >

200 °C. Further, in embodiments, the oxidation temperature Tox may be equal to or higher than the reduction temperature Tred, such that Tox > Trea. Alternatively, in embodiments, the oxidation temperature Tox may especially be equal to or lower than the reduction temperature

Tred, such that Tox < Trea. Hence, in specific embodiments, Tred - Tox < 25 °C.

Further, in embodiments, the reduction temperature Treq may be selected from the range of < 950 °C, such as < 850 °C, especially < 750 °C. Analogously, in embodiments, the oxidation temperature Tox may be selected from the range of < 950 °C, such as < 850 °C, especially <750 °C.

In embodiments, the method, especially the reduction stage, may comprise providing the reduction gas 410 to the reactor 200 at a pressure Prea. Additionally, the method, especially the oxidation stage, may comprise providing the oxidation gas 420 to the reactor 200 at a pressure Pox. In such embodiments, 0 bar < Poy - Prea < 60 bar, such as 0 bar < Poy - Pred < 45 bar, especially 1 bar < Poy - Pred < 45 bar, more especially 1 bar < Pox - Prea < 30 bar.

Especially, in embodiments, Pox - Prea > 1 bar.

In embodiments, the method may comprise alternating between the reduction stage and the oxidation stage. Hence, in embodiments, the reduction stage may have a reduction duration freq selected from the range of 5-150 min, such as 10-120 min, especially 10-90 min.

Further, in embodiments, the oxidation stage may have an oxidation duration fox selected from the range of 5-150 min, such as 10-120 min, especially 10-90 min. Hence, the oxidation stage and the reduction stage may have an equal duration. However, in embodiments, the oxidation stage and the reduction stage may have a different duration.

Further, in embodiments, the invention may provide a use of the redox material 110 for providing hydrogen from water vapor. In such embodiments, the redox material 110 may comprise a nanostructure | selected from the group comprising a nanorod 10 and a nanowire 20. Further, in such embodiments, the redox material 110 may comprise > 25 wt%, such as > 40 wt%, especially > 50 wt% cerium dioxide. In embodiments, the redox material 110 may further comprise 0-50 wt%, such as 1-40 wt%, especially 1-25 wt% iron oxide.

Fig. 2A schematically depicts an embodiment of the reactor material 100. In embodiments, the reactor material 100 may comprise the redox material 110. Further, in embodiments, the reactor material may comprise a support material 120. In such embodiments, the support material 120 may be configured to support the redox material 110. As depicted in

Fig. 2, the redox material 110 may especially be dispersed throughout the support material 120.

In embodiments, the support material 120 may comprise a thermally stable and non-reactive material. Particularly, in embodiments, the support material 120 may be selected from the group comprising alumina, silica, titania, and zirconia. In certain embodiments, the reactor material 100 may comprise at least 40%, such as at least 55%, especially at least 70%, more especially at least 80% support material 120. Further, in embodiments, the reactor material 100 may comprise at most 70 wt%, such as at most 55 wt%, especially at most 35 wt% iron oxide.

Additionally or alternatively, in embodiments, the redox material 110 may comprise 0-50 wt®%, such as 1-35 wt%o, especially 3-20 wt% iron oxide.

In embodiments, the nanostructure 1 may be the nanorod 10. In such embodiments, the nanorod 10 may have a length L and an equivalent circular diameter D individually selected from the range of 1-500 nm. In embodiments, the length L may further be selected from the range of 5-400 nm, such as 5-300 nm, especially 10-200 nm. Additionally, in embodiments, the equivalent circular diameter D may be selected from the range of 1-100 nm, such as 1-50 nm, especially 1-20 nm.

In embodiments, the redox material may (also) comprise a nanorod bundle 11.

In embodiments, the nanorod bundle 11 may comprise at least two nanorods 10. In such embodiments, the at least two nanorods 10 may be (reversibly) physically connected.

Especially, in embodiments, at least one nanorod 10 of the at least two nanorods 10 may be adsorbed onto a side of another of the at least two nanorods 10. Further, in embodiments, the nanorod bundle 11 may comprise at least 5 nanorods 10. Further still, in embodiments, the nanorod bundle 11 may comprise at least 10 nanorods 10, such as at most 30 nanorods 10. In embodiments, the nanorods 10 within the nanorod bundle 11 may be oriented (essentially) in parallel, as depicted in Fig. 2. Yet, in alternative embodiments, the nanorods 10 within the nanorod bundle 11 may be oriented in random directions.

Fig. 2B schematically depicts another embodiment of the reactor material 100.

In embodiments, the nanostructure 1 may be the nanowire 20. In such embodiments, the nanowire 20 may have a length L and an equivalent circular diameter D. In embodiment, the length L of the nanowire 20 may be selected from the range of 100-2000 nm, such as 100-1500 nm, especially 100-1000 nm. In embodiments, the length L may be selected from the range of 100-500 nm. Additionally, in embodiments, the equivalent circular diameter D may be selected from the range of 1-100 nm, such as 1-50 nm, especially 1-20 nm. In further embodiments, the equivalent circular diameter D may be selected from the range of 2-10 nm. In embodiments, the nanowire 20 may have a deviation in the equivalent circular diameter D of < 15%, such as < 10%, especially < 5% over at least 60%, such as at least 70%, especially at least 80% of the length L of the nanowire 20.

Fig. 3 depicts an embodiment of a redox material 110 comprising a nanostructure 1, especially a nanorod 10. Specifically, in the depicted embodiment, the redox material 110 comprises a plurality of nanostructures 1, especially a plurality of nanorods 10. In embodiments, the nanostructure 1, especially the nanorod 10, may comprise a crystalline material. In other embodiments, the nanostructure 1, especially the nanorod 10, (itself) may be (essentially) crystalline. Further, in embodiments, the nanostructure 1, especially the nanorod 10, may comprise one or more sides. The one or more sides may, in embodiments, comprise a crystal plane, selected from the group comprising {001}, {100}, {110}, and {111} crystal planes, as denoted in Miller indices. Hence, in specific embodiments, at least one of the one or more sides of the nanostructure 1, especially the nanorod 10, may comprise {110} and {100} crystal planes.

Experiments

Embodiments of the method of the invention were experimentally evaluated with different reactor materials 100, redox materials 110, and conditions during the reduction stage and/or the oxidation stage. These experiments are briefly described herein.

Unless specified otherwise, the experiments were performed using the following conditions.

Chemicals — dry argon was selected as the reduction gas 410, with a flow rate of 21.4 mL/min. The oxidation gas 420 consisted of argon, with a flow rate of 6.13 mL/min, and water vapor, wherein the mole fraction of water in the oxidation gas 420 was 0.669. A syringe pump was used to supply water at a controlled flow rate of 0.01 mL/min to the T- junction of an electrically heated coil (at 110 °C), where it mixed with dry argon gas to generate the oxidation gas 420. The reactor material 100 comprised (doped) ceria nanorods 10 with lengths L 30-45 nm and widths 5-10 nm, without a support material 120. The reactor material 100 was first pelletized and sieved to form particle of sizes between 180 and 300 um to reduce pressure drop in the reactor 200.

Apparatus — the gas supply 400 was equipped with a four-way valve to regulate the gas flow of the reduction gas 410 and the oxidation gas 420 to the reactor 200. The flowrates of the reduction gas 410 and the oxidation gas 420 were controlled separately by two flowrate controllers. The reactor 200 was equipped with a quartz tube, into which the reactor material 100 was placed.

Process parameters — Both the reduction duration tse: and the oxidation duration tox were set to 90 minutes. About 200 mg of reactor material pellets were carefully packed in a cm quartz tube with an inner diameter of 4 mm. First, a bit of quartz wool was rolled and pushed into the tube, followed by loading of the reactor material pellets, and another piece of quartz wool was used to seal in the reactor material pellets, forming a reactor bed length of ~ 1 cm. The quartz tube was placed in the reactor 200 and secured tightly with bolts and ferrules.

The system 1000 was tested for leaks using a soap solution. Additionally, a flow detector was used at the reactor outlet 220 to confirm that the correct gas flow was obtained. The reduction gas pressure Preg and the oxidation gas pressure Pox were set to ~1 bar (atmospheric pressure).

The switch from reduction gas 410 to oxidation gas 420 and vice versa typically occurred after a stable temperature had been maintained for a specified duration.

Analysis — hydrogen production was measured at the reactor outlet 220 using a gas chromatograph (GC) and a quadrupole mass spectrometer (MS). Prior to analysis, the gases at the reactor outlet 220 were passed through a condenser and water trap before being directed towards the analytical system (GC and MS). The hydrogen production was quantified as the amount (in mL) of hydrogen produced per gram of redox material 110. Using a GC calibration curve, peak area values from the raw data were converted to percentage of hydrogen produced over time. The hydrogen production rate in mL/min was calculated by converting the percentage values using the known flowrates. The total hydrogen production was quantified by integrating the hydrogen production rate curve areas over time for each cycle and summing over all cycles. Finally, the total value was divided over the amount of redox material 110 as a standard for quantification. The MS data was used for online monitoring of the different component flows during the reaction.

Experiment 1 — Effect of iron oxide doping on hydrogen production

A reactor material 100 comprising the nanorods 10 as described above, with different amounts of iron oxide doping, was placed in the reactor 200 as described above. The reactor material 100 was exposed to three reduction/oxidation cycles at various reduction temperatures Tred and oxidation temperatures Tox, using the chemicals, apparatus, and process parameters provided above. The hydrogen production per reduction/oxidation cycle was determined, as well as the combined hydrogen production over the three cycles. The results of these experiments can be found in Table 1.

Table 1. Hydrogen production for nanorods 10 under various conditions.

Sample and Composition | Temperature Hydrogen production [mw [ow pm Jew

Undoped ceria nanorods 700,700 [6246 [0 [0 [6246

Too [ae fo [ofa wo [aw 0 Jo Jaws

Iron-doped ceria nanorods | 700, 700 8.916 0.252 0.036 9.204 emmae)

Iron-doped ceria nanorods (10 wt% iron oxide in CeO»)

Iron-doped ceria nanorods | 700, 700 10.003 | 1.475 0.309 11.786

Ee ee

As can be seen from table 1, experiments performed with iron-doped ceria nanorods have an improved combined hydrogen production over undoped ceria nanorods, wherein nanorods doped with 10 wt% iron oxide outperform those doped with 5 wt% and 25 wt%. As can further be seen from table 1, the use of lower oxidation temperatures Tox and/or reduction temperatures Tres may result in a higher hydrogen generation in the second and third cycles, which may further apply to potential additional cycles.

The term “plurality” refers to two or more. Furthermore, the terms “a plurality of” and “a number of” may be used interchangeably.

The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%.

Moreover, the terms “about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”,

“about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

The term “comprise” also includes embodiments wherein the term “comprises” means “consists of”.

The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of" but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species”.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

Conclusions

1. A method for providing hydrogen using a reactor material (100) comprising a redox material (110), wherein the redox material (110) comprises a nanostructure (1) selected from the group consisting of a nanorod (10) and a nanowire (20), and wherein the redox material (110) comprises >50 wt% cerium dioxide, and wherein the redox material (110) comprises 1-25 wt% iron oxide, the method comprising: - a reduction step comprising exposing the redox material (110) to a reduction temperature Trea and a reduction gas (410), wherein the reduction temperature Trea is selected from the range of >300°C, and wherein the reduction gas (410) comprises an inert gas; - an oxidation step comprising exposing the redox material (110) to an oxidation gas (420), wherein the oxidation gas (420) comprises water vapor.

The method of claim 1, wherein the nanostructure (1) comprises the nanorod (10), wherein the nanorod (10) comprises >50 wt% cerium dioxide, and wherein the nanorod (10) comprises 1-25 wt% iron oxide.

The method according to any one of the preceding claims, comprising exposing the redox material (110) to an oxidation temperature Tox in the oxidation phase, wherein the oxidation temperature Tox is selected from the range of > 200 °C, and wherein Tox < Tred. A

4 The method according to claim 3, wherein the reduction temperature Tred is selected from the range of < 750 °C 1s, and wherein the oxidation temperature Tox is selected from the range of < 750 °C.

5. The method according to any one of the preceding claims 3-4, wherein Treg Tok <25 °C.

6. The method according to any one of the preceding claims, comprising supplying the reducing gas (410) to the redox material (110) at a pressure Preg, and supplying the oxidation gas (420) to the reactor (200) at a pressure Po, wherein 1 bar < Pox - Prea < 30 bar.

The method of any preceding claim, wherein the reactor material (100) comprises a support material (120), the support material (120) configured to support the redox material (110), the support material (120) comprising a thermally stable and non-reactive material, and the reactor material (100) comprising at most 35 wt% iron oxide.

8. The method of any preceding claim, wherein the redox material (110) comprises 7 - 13 wt% iron oxide.

9. The method according to any one of the preceding claims 2-8, wherein the nanorod (10) has a length (L) selected from the range of 10-200 nm and an equivalent circular diameter (D) selected from the range of 1-20 nm.

10. The method according to any one of the preceding claims, wherein the reduction phase has a reduction time (free) selected from the range of 10-90 min, and wherein the oxidation phase has an oxidation time (fox) selected from the range of 10-90 min.

11. The method according to any one of the preceding claims, wherein the method comprises alternating the reduction phase and the oxidation phase.

The method of any preceding claim, wherein the redox material (110) comprises a nanorod bundle (11), the nanorod bundle (11) comprising at least two nanorods (10), the at least two nanorods (10) being physically connected.

13. The method of claim 12, wherein the nanorod bundle (11) comprises at least 5 nanorods (10).

14. The method of any preceding claim, wherein the nanostructure (1) is crystalline, wherein the nanostructure (1) comprises one or more sides, and wherein at least one of the one or more sides of the nanostructure (1) comprises {110} and {100} crystal planes.

15. A system (1000) for providing hydrogen, the system comprising a reactor (200), a temperature control unit (500) and a gas supply (400), the reactor (200) configured to host a reactor material (100) comprising a redox material (110), the redox material (110) comprising a nanostructure (1) selected from the group consisting of a nanorod (10) and a nanowire (20), and the redox material (110) comprising >50 wt% cerium dioxide, and the redox material (110) comprising 1-25 wt% iron oxide, the system configured to switch between a reduction phase and an oxidation phase, wherein: - in the reduction phase, the gas supply (400) is configured to provide a reduction gas (410) to the reactor (200), the reduction gas (410) comprising an inert gas, and wherein in the reduction phase the temperature control unit (500) is configured to control a reduction temperature Tred of the reactor (200), the reduction temperature Trey being selected from the range > 300 °C; - in the oxidation phase the gas supply (400) is configured to provide an oxidation gas (420) to the reactor (200), the oxidation gas (420) comprising water vapour.

16. The system (1000) of claim 15, wherein in the oxidation phase the temperature control unit (500) is configured to control an oxidation temperature Tox of the reactor (200), the oxidation temperature Toy being selected from the range of > 200 °C, and Tox < Trea.

17. The system (1000) of any one of claims 15 to 16, wherein the reduction temperature Trea < 750 °C, and wherein the oxidation temperature Tox < 750 °C.

The system (1000) of any of claims 15 to 17, wherein in the reduction phase the gas supply (400) is configured to provide the reduction gas (410) to the reactor (200) at a reduction gas pressure Pred, and wherein in the oxidation phase the gas supply (400) is configured to provide the oxidation gas (420) to the reactor (200) at an oxidation gas pressure Pox, wherein 1 bar < Pox — Pred < 30 bar.

The system (1000) of any of claims 15-18, wherein the reactor material (100) further comprises a support material (120), wherein the redox material (110) is dispersed in the support material (120), and wherein the reactor material (100) comprises at most 35 wt% iron oxide.

20. The system (1000) of any one of the preceding claims 15-19, wherein the system comprises an operating system (300), the operating system (300) configured to perform the method of any one of the preceding claims 1-13.

21. Use of a redox material (110) for providing hydrogen from water vapor, wherein the redox material (110) comprises a nanostructure (1) selected from the group consisting of a nanorod (10) and a nanowire (20), wherein the redox material (110) comprises >50 wt% cerium dioxide, wherein the redox material (110) comprises 1-25 wt% iron oxide.