TRANSITION METAL MXENE CATALYSTS FOR CONVERSION OF CARBON DIOXIDE TO HYDROCARBONS

Abstract

Transition metal MXene catalysts and methods for using with electrochemical cells for reduction of carbon dioxide and production of hydrocarbons. The transition metal catalysts include nanostructured transition metal carbides, nitrides, or carbonitrides. The method includes electrochemically reducing carbon dioxide in an electrochemical cell, by contacting the carbon dioxide with at least one transition metal carbide, nitride, or carbonitride catalyst in the electrochemical cell and applying a potential to the electrochemical cell. Also an apparatus and method for energy production and carbon sequestration. A photovoltaic cell is paired with an electrochemical cell, wherein a cathode side of the electrochemical cell reduces carbon dioxide to hydrocarbon, and an anode side of the electrochemical cell oxidizes water to oxygen. The hydrocarbon outlet can be connected to a heating element of an air handling unit, and the oxygen can likewise be introduced to the unit for air improvement. The cathode includes transition metal catalysts for reducing the carbon dioxide.

Claims

1. A method of electrochemically reducing carbon dioxide, comprising: introducing the carbon dioxide to a catalyst comprising a transition metal carbide, nitride, or carbonitride catalyst coated on a cathode and in contact with a liquid catholyte in an electrochemical cell, wherein the nanostructured transition metal carbide comprises a plurality of nanoparticles, nanoflakes, and/or nanoribbons, having an average structure size between about 1 nm and 400 nm; applying a potential to the electrochemical cell; and converting the carbon dioxide to a hydrocarbon.

2. A method of claim 1, further comprising a co-catalyst system comprising an imidazolium-functionalized transition metal carbide, nitride, or carbonitride.

3. A method of claim 1, further comprising: providing carbon dioxide to the electrochemical cell; and applying the potential to the electrochemical cell in the presence of the carbon dioxide to reduce the carbon dioxide to the hydrocarbon.

4. The method of claim 3, further comprising an electrolyte of a solution of 1 M KHCO.sub.3 or 1 M KOH, and saturating the electrolyte with the carbon dioxide.

5. A method of claim 1, wherein the catalyst comprises a nanostructured MXene.

6. A method of claim 5, wherein the catalyst comprises M.sub.yX.sub.z, wherein M is a transition metal, X is carbon and/or nitrogen, and y and z are stoichiometric ratio integers.

7. A method of claim 1, wherein the transition metal comprises molybdenum, tungsten, titanium, or cobalt.

8. A method of claim 1, wherein the catalyst comprises a nanoparticle form.

9. A method of claim 1, wherein the catalyst comprises a nanostructured bimetallic or trimetallic MXene carbide, nitride, or carbonitride.

10. A method of claim 9, wherein the catalyst comprises a formula of M.sup.1xM.sup.2yXz or M.sup.1wM.sup.2xM.sup.3yXz, wherein each of M.sup.1, M.sup.2, and M.sup.3 is a different metal type, X is carbon, nitrogen or carbonitride, and each of w, x, y, and z is a stoichiometric ratio integer.

11. A method of claim 9, wherein the hydrocarbon is a C2, C3, C4, or C5 hydrocarbon.

12. A method of claim 9, wherein the catalyst comprises a bimetallic or trimetallic copper-based alloy.

13. A method of claim 12, wherein the catalyst comprises a tungsten-copper carbide, nitride, or carbonitride.

14. A method of claim 12, wherein the catalyst comprises a formula of W.sub.2−xCu.sub.xC, W.sub.2−xCu.sub.xN, W.sub.2−x−yCu.sub.x M.sub.yC, W.sub.2−x−yCu.sub.x M.sub.yN or W.sub.2−x−yCu.sub.x M.sub.yCN, where M is Ce, Zn, Ta, Fe, Sc, Zr, Mn, Cd, La, Cr, or Sn, with 0.1≤X≤1 and 0.1<y<0.5.

15. A method of claim 9, wherein the catalyst comprises a functionalized surface modification.

16. The method of claim 15, wherein the catalyst comprises a surface modified with defects and/or modified with alkali and earth alkali metals and halides, oxygen, phosphorus, sulfur, selenium, tellurium, and/or hydroxide.

17. A method of claim 9, further comprising an electrolyte without water or a buffer.

18. A method of claim 1, wherein the catalyst comprises a functionalized surface modification.

19. A method of claim 1, further comprising an electrolyte without water or a buffer.

20. An electrochemical cell having a cathode with at least one bimetallic or trimetallic MXene catalyst, and in contact with an electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic sectional view of an electrochemical device according to one embodiment of this invention.

[0022] FIG. 2 representatively illustrates a two-compartment three-electrode electrochemical cell according to one embodiment of this invention.

[0023] FIG. 3 is a general schematic, with a partial sectional view of an electrochemical device, according to a system of one embodiment of this invention.

DESCRIPTION OF THE INVENTION

[0024] This invention relates generally to reduction of carbon dioxide (CO.sub.2) to hydrocarbons such as methane (CH.sub.4) and, more particularly, to MXene materials as catalysts for this reduction.

[0025] The invention provides transition metal catalysts and method of using the catalysts to reduce carbon dioxide, such as to hydrocarbons for use as fuel. Exemplary catalysts include nanostructured MXenes, such as typically in one of the following structures: M.sub.2X (e.g., M.sub.2N, M.sub.2C, or M.sub.2CN), M.sub.3X.sub.2 (e.g., M.sub.3N.sub.2, M.sub.3C.sub.2, or M.sub.3C.sub.2N), and M.sub.4X.sub.3 (e.g., M.sub.4C.sub.3N), wherein M is a transition metal and X is carbon, nitrogen, or a carbonitride. The invention further includes a nanostructured bimetallic or trimetallic MXene carbide, nitride, or carbonitride, such as M.sup.1xM.sup.2yXz or M.sup.1wM.sup.2xM.sup.3yXz, wherein M.sup.1, M.sup.2, and M.sup.3 are each different metal types, desirably including one or more different types of transition metals, X is carbon, nitrogen or carbonitride, and w, x, y, and z are each stoichiometric ratio integers. One presently preferred transition metal is molybdenum, such as in the form of Mo.sub.2C or Mo.sub.2CN nanoparticles or nanoflakes. Other exemplary MXenes include, without limitation, carbides, nitrides, or carbonitrides of cobalt, titanium, tungsten, etc. Multiple metals and/or multiple stoichiometries are also possible for the MXene catalysts.

[0026] Two ultimate goals in the electrochemical reduction of carbon dioxide can be addressed by using the transition metal MXene catalysts of this invention. First, it tackles the amount of required energy to reduce the CO.sub.2 into useful products. The observed onset overpotential for the CH.sub.4 formation (−0.15 V vs. RHE) using Mo.sub.2C is the lowest reported to date which shows its superior catalytic activity among commonly used catalysts. Second, employing Mo.sub.2C catalysts provides production of CH.sub.4 having two orders of magnitude higher numbers of product formation compared to typical state of the art metal catalysts (e.g., copper).

[0027] FIG. 1 is a schematic sectional view of an electrochemical device 20 (e.g., electrochemical cell) with a first compartment 22 including at least one transition metal MXene 24 disposed on a cathode 26. Device 20 includes a second compartment 32 including at least one water oxidizing catalyst 34 disposed on an anode 36. Compartments 22 and 32 include a first electrolyte 28 and a second electrolyte 38, respectively, and are in ionic contact through an ion-conductive membrane 40. An electrical potential source 50 is included. In embodiments of this invention, the electrical potential source is a photovoltaic cell. The device 20 further includes a carbon dioxide inlet and a suitable hydrocarbon outlet, and a corresponding anode side inlet and outlet.

[0028] The transition metal MXene catalysts of embodiments of this invention have a general chemical formula of M.sub.yX.sub.z, wherein M is a transition metal, X is carbon and/or nitrogen, and y and z are stoichiometric ratio integers (generally each one of 1-4, with y and z being equal or y one whole number greater than z; e.g., M.sub.2X, M.sub.3X.sub.2, and/or M.sub.4X.sub.3). In embodiments of this invention, the catalyst is or includes M.sub.n+1X.sub.n, wherein M is a transition metal, X is carbon and/or nitrogen, and n is zero or an integer. In additional embodiments of this invention, the catalyst is or includes M.sub.xC.sub.yN.sub.z wherein M is a transition metal, C is carbon, N is nitrogen, and x, y and z are each an stoichiometric ratio integer (e.g., with each of y and z being independently one of 0 to 3, with at least one of y and z not zero, and x, y and/or z being equal or x being one whole number greater than y or z (e.g., MC, MN, M.sub.2C, M.sub.2N, M.sub.3C.sub.2, M.sub.4C.sub.3, M.sub.2CN, M.sub.3C.sub.2N, and/or M.sub.4C.sub.3N). Presently preferred transition metals include molybdenum, tungsten, titanium, or cobalt. Exemplary catalyst materials include, without limitation, WC, TiC, Co.sub.2C, and/or Mo.sub.2C.

[0029] The transition metal MXene catalysts of embodiments of this invention further includes alloyed catalysts, having two or more different types of metals bonded within the alloy. The alloy catalysts of this invention are particularly suited for producing hydrocarbons other than methane. Embodiments of this invention provide and use multinary metallic atoms with various stoichiometry bonding with carbon and/or nitrogen such as MoxCuyZnzC. These materials, particularly with further modifications in the structure, promote C—C bonding and allow for production of C1-C5 hydrocarbons, and preferably C2, C3, C4, or C5 hydrocarbons.

[0030] The transition metal catalyst can include a nanostructured bimetallic or trimetallic MXene carbide, nitride, or carbonitride, such as M.sup.1xM.sup.2yXz or M.sup.1wM.sup.2xM.sup.3yXz, wherein M.sup.1, M.sup.2, and M.sup.3 are each different metal types, desirably including one or more different types of transition metals, X is carbon, nitrogen or carbonitride, and w, x, y, and z are each stoichiometric ratio integers. Alloyed catalysts of this invention (e.g., M.sup.1, M.sup.2, and/or M.sup.3) can include any combination of the following transition metals with carbide (C), nitride (N), or carbonitride (CN), with various stoichiometry: Ce, Sc, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd.

[0031] As a presently preferred example, the invention includes a bimetallic or trimetallic copper-based alloy. As an example, the invention includes a tungsten-copper alloy catalyst of the formula W.sub.2−xCu.sub.xC (0.1≤X≤1). The invention further includes trimetallic and high entropy alloy catalysis such as W.sub.2−x−yCu.sub.xM.sub.yC, where M is: Ce, Zn, Ta, Fe, Sc, Zr, Mn, Cd, La, Cr, or Sn, with 0.1≤X≤1 and 0.1<y<0.5. This combination can be used for nitride and carbonitride materials, for example, W.sub.2−xCu.sub.xN or W.sub.2−xCu.sub.xCN, and W.sub.2−x−yCu.sub.x M.sub.yN or W.sub.2−x−yCu.sub.x M.sub.yCN.

[0032] In embodiments of this invention, the nanostructured carbide, nitride, or carbonitride catalysts are further modified to promote catalytic activity and active sites, such as with surface defects or other modifications. Exemplary modifications include, without limitation, defective sites, doped structures or other surface functionalized structures, such as modified with alkali and earth alkali metals and halides, oxygen, phosphorus, sulfur, selenium, tellurium, and/or hydroxide

[0033] Embodiments of the invention incorporate electrolytes that do not involve water or a buffer. Exemplary electrolytes include highly alkaline solutions of KOH, CsOH, or NaOH, preferably about 0.5-10 M. Acidic electrolytes without water or buffer may also be used in particular instances.

[0034] The transition metal MXene catalysts can be provided in a variety of forms, for example, as a bulk material, in nanostructure form, as a collection of particles, and/or as a collection of supported particles. The MXene catalyst in bulk form can have a layered structure as is typical for such compounds. The MXene catalyst may have a nanostructure morphology, including but not limited to monolayers, nanotubes, nanoparticles, nanoflakes (e.g., multilayer nanoflakes), nanosheets, nanoribbons, nanoporous solids, etc. As used herein, the term “nanostructure” refers to a material with a dimension (e.g., of a pore, a thickness, a diameter, as appropriate for the structure) in the nanometer range.

[0035] In some embodiments, the catalyst is a layer-stacked bulk MXene with metal atom-terminated edges. In other embodiments, MXene nanoparticles may be used in the devices and methods of the disclosure. In other embodiments, al MXene nanoflakes may be used in the devices and methods of the disclosure. Nanoflakes can be made, for example, via liquid exfoliation, as described in Coleman, J. N. et al., “Two-dimensional nanosheets produced by liquid exfoliation of layered materials.” Science 331, 568-71 (2011) and Yasaei, P. et al., “High-Quality Black Phosphorus Atomic Layers by Liquid-Phase Exfoliation.” Adv. Mater. (2015) (doi:10.1002/adma.201405150), each of which is hereby incorporated herein by reference in its entirety. In other embodiments, transition metal MXene nanoribbons may be used in the devices and methods of the disclosure. In other embodiments, transition metal MXene nanosheets may be used in the devices and methods of the disclosure. The person of ordinary skill in the art can select the appropriate morphology for a particular device.

[0036] In some embodiments of the methods and devices as otherwise described herein, the transition metal MXene nanostructures (e.g., nanoflakes, nanoparticles, nanoribbons, etc.) have an average size between about 1 nm and 1000 nm. The relevant size for a nanoparticle is its largest diameter. The relevant size for a nanoflake is its largest width along its major surface. The relevant size for a nanoribbon is its width across the ribbon. The relevant size for a nanosheet is its thickness. In some embodiments, the transition metal MXene nanostructures have an average size between from about 1 nm to about 400 nm, or about 1 nm to about 350 nm, or about 1 nm to about 300 nm, or about 1 nm to about 250 nm, or about 1 nm to about 200 nm, or about 1 nm to about 150 nm, or about 1 nm to about 100 nm, or about 1 nm to about 80 nm, or about 1 nm to about 70 nm, or about 1 nm to about 50 nm, or 50 nm to about 400 nm, or about 50 nm to about 350 nm, or about 50 nm to about 300 nm, or about 50 nm to about 250 nm, or about 50 nm to about 200 nm, or about 50 nm to about 150 nm, or about 50 nm to about 100 nm, or about 10 nm to about 70 nm, or about 10 nm to about 80 nm, or about 10 nm to about 100 nm, or about 100 nm to about 500 nm, or about 100 nm to about 600 nm, or about 100 nm to about 700 nm, or about 100 nm to about 800 nm, or about 100 nm to about 900 nm, or about 100 nm to about 1000 nm, or about 400 nm to about 500 nm, or about 400 nm to about 600 nm, or about 400 nm to about 700 nm, or about 400 nm to about 800 nm, or about 400 nm to about 900 nm, or about 400 nm to about 1000 nm.

[0037] In certain embodiments of the methods and devices as otherwise described herein, transition metal MXene nanoflakes have an average thickness between about 1 nm and about 100 μm (e.g., about 1 nm to about 10 μm, or about 1 nm to about 1μm, or about 1 nm to about 1000 nm, or about 1 nm to about 400 nm, or about 1 nm to about 350 nm, or about 1 nm to about 300 nm, or about 1 nm to about 250 nm, or about 1 nm to about 200 nm, or about 1 nm to about 150 nm, or about 1 nm to about 100 nm, or about 1 nm to about 80 nm, or about 1 nm to about 70 nm, or about 1 nm to about 50 nm, or about 50 nm to about 400 nm, or about 50 nm to about 350 nm, or about 50 nm to about 300 nm, or about 50 nm to about 250 nm, or about 50 nm to about 200 nm, or about 50 nm to about 150 nm, or about 50 nm to about 100 nm, or about 10 nm to about 70 nm, or about 10 nm to about 80 nm, or about 10 nm to about 100 nm, or about 100 nm to about 500 nm, or about 100 nm to about 600 nm, or about 100 nm to about 700 nm, or about 100 nm to about 800 nm, or about 100 nm to about 900 nm, or about 100 nm to about 1000 nm, or about 400 nm to about 500 nm, or about 400 nm to about 600 nm, or about 400 nm to about 700 nm, or about 400 nm to about 800 nm, or about 400 nm to about 900 nm, or about 400 nm to about 1000 nm); and average dimensions along the major surface of about 20 nm to about 100 μm (e.g., about 20 nm to about 50 μm, or about 20 nm to about 10 μm, or about 20 nm to about 1 μm, or about 50 nm to about 100 μm, or about 50 nm to about 50 μm, or about 50 nm to about 10 μm, or about 50 nm to about 1 μm, or about 100 nm to about 100 μm, or about 100 nm to about 50 μm, or about 100 nm to about 10 μm, or about 100 nm to about 1 μm), The aspect ratio (largest major dimension:thickness) of the nanoflakes can be on average, for example, at least about 5:1, at least about 10:1 or at least about 20:1. For example, in certain embodiments the transition metal MXene nanoflakes have an average thickness in the range of about 1 nm to about 1000 nm (e.g., about 1 nm to about 100 nm), average dimensions along the major surface of about 50 nm to about 10 μm, and an aspect ratio of at least about 5:1.

[0038] The invention includes methods of electrochemically reducing carbon dioxide by introducing the carbon dioxide to a transition metal MXene catalyst in an electrochemical cell. Embodiments of this invention utilize nanostructured transition metal MXenes as catalysts in the electrocatalytic conversion of carbon dioxide (CO.sub.2) to produce hydrocarbon, such methane (CH.sub.4), the main component of natural gas, at remarkably low overpotentials.

[0039] The nanostructured transition metal MXenes can be synthesized using liquid exfoliation techniques, and were tested in a two-compartment three-electrode electrochemical cell as a working electrode, as shown in FIG. 2. FIG. 2 representatively illustrates the two-compartment three-electrode electrochemical cell according to embodiments of this invention used for testing. Transition metal carbides were drop-cast onto a glassy carbon substrate to form the working electrode 124. Platinum gauze or other suitable material can be used as the counter and reference electrodes 136 and 126, respectively. The working electrode 124, reference electrode 126, and counter electrode 136 are immersed in an aqueous electrolyte solution 128 and 138, respectively. The cathode and anode are separated by an ion-conductive membrane 140 to eliminate potential product oxidation at the anode 136 surface.

[0040] Testing results indicated that Mo.sub.2C exhibited an onset potential of −0.15 V vs. RHE, which is a potential where the reduction reaction begins in a buffer electrolyte of 1 M KHCO.sub.3. The recorded onset potential for Mo.sub.2C is the lowest overpotential (−0.15 V), excess energy beyond thermodynamic potential, for CH.sub.4 formation reported so far, which is 650 mV less than that of copper (−0.8 V). Mo.sub.2C also exhibits significantly higher faradaic efficiency at a potential range of −0.15 to −0.8 V. For instance, at a potential of −0.4 V, methane formation F.E. for Mo.sub.2C nanoflake is 44% while copper has a negligible faradaic efficiency of less than 1%. Moreover, the calculated turnover frequency (TOF), the number of product (CH.sub.4) formation per active sites, for Mo.sub.2C indicated approximately two orders of magnitude higher CH.sub.4 formation than that of copper at a potential range of −0.15 to −0.8 V vs. RHE.

[0041] Thus, the invention provides a method and system to recycle CO.sub.2 into hydrocarbons, such as CH.sub.4 (natural gas) in an energy efficient and economically feasible electrochemical process. A scale-up of the invention coupled with solar energy cells can develop a carbon-zero electrochemical system in which CO.sub.2 from the air, wastes of the big industries, etc. can be reduced to a profitable product (natural gas) that can directly be used as a fuel.

[0042] This invention includes a method and system for carbon sequestration and/or energy production, implemented for or as outer surfaces of structures, such as residential and/or commercial building structures. Embodiments of this invention provide a cladding, or building skin, that acts as an artificial leaf to achieve at least two objectives simultaneously: a) produces energy for operation of buildings, and b) sequesters carbon through chemical processes. The invention includes a façade cladding system that not only contributes to traditional aesthetical, thermal, and structural roles of building skins, but also is able to approach PV panels in terms of efficiency in energy generation, and at the same time absorb carbon dioxide (which is a major cause of global warming).

[0043] Embodiments of this invention provide and/or incorporate an artificial leaf system which combines sunlight, carbon dioxide, and water to generate energy in an artificial photosynthesis process, adjusted with new materials and chemical processes to improve efficiency with regard to energy production and carbon sequestration. This system is integrated into a building cladding or skin (such as opaque external walls) to convert façade cladding systems into energy-generator/carbon-removal systems.

[0044] This invention also includes methods of electrochemically reducing carbon dioxide by introducing the carbon dioxide to one or more reduction catalysts in an electrochemical cell. Embodiments of this invention utilize nanostructured catalysts, such as transition metal catalysts, in the electrocatalytic conversion of carbon dioxide (CO.sub.2) to produce hydrocarbons such as methane (CH.sub.4), the main component of natural gas, at low overpotentials.

[0045] FIG. 3 generally illustrates an artificial leaf-based façade cladding system according to embodiments of this invention. FIG. 3 shows a representative building structure 120 including outer surface claddings 122. The claddings can be sized and shaped as needed for any particular building structure. The claddings can also be incorporated in various and alternative placements on the building, such as near or along a top edge of an outer wall, on a rooftop or structure thereon, or integrated in, between and/or around windows, design elements, etc.

[0046] FIG. 3 shows a sectional view of one of the claddings 122. The cladding 122 includes an energy source, preferably a photovoltaic cell 124 on a side, e.g., an outer side facing the sun light 126, of a system support housing 128. Any suitable photovoltaic cell can be used, depending on need, such as a triple junction photovoltaic (3j-PV) cell. The photovoltaic cell can provide solar energy to the grid, the building, and/or the cladding itself.

[0047] The support housing further includes, contains, or otherwise supports an electrochemical cell 130 for reducing carbon dioxide, desirably using energy potential from the photovoltaic cell 124. The electrochemical cell 130 includes a cathode side 132 with a cathode 134 partitioned from an anode side 142 with anode 144. The cathode side 132 includes a suitable catholyte 136 in ionic contact with a suitable anolyte 146 of the anode side 142 by a suitable ion-conductive membrane 140. The catholyte and anolyte can be a solution, such as separated by a suitable ion-conductive membrane 140, or a solid or semi-solid polymer based electrolyte.

[0048] The cathode side 132 includes a catalyst 138 that, together with liquid catholyte 136 (e.g., imidazolium) as a co-catalyst system, reduces carbon dioxide (CO.sub.2) to a hydrocarbon, such as methane (CH.sub.4). The catalyst 138 can further be a functionalized co-catalyst, such as a co-catalyst including an imidazolium-functionalized transition metal carbide, nitride, or carbonitride of the types discussed below. The anode side 142 can include a second catalyst 148, for example, a cobalt catalyst, which co-currently oxidizes water in the anolyte solution 146 into oxygen without applying any external potential. The catalysts, discussed further below, can be any suitable catalyst, and are desirably coated or otherwise contained on, in, or at, the cathode 134 or anode 144, respectively.

[0049] Carbon dioxide is introduced through a carbon inlet to the cathode side 132 and is reduced to methane collected and released through a carbon outlet. Similarly, water introduced to the anode side 142 results in oxygen gas released through an oxygen outlet. The collected hydrocarbon and/or oxygen can be collected or delivered as desired. For example, the anode and/or cathode can include suitable perforations or pores to provide paths for the gas transfer to and from the catalysts. Additionally or alternatively, any suitable piping and valves can be used to provide and/or control the carbon dioxide, water, oxygen, and/or hydrocarbon.

[0050] In FIG. 3, the methane is delivered through a conduit into a heating system 150 of the building 120. The oxygen is released to the environment, and preferably the building interior through the building air handler unit (AHU) 152 of the heating, ventilation, and air conditioning system to purify air by providing the fresh O.sub.2 into the building.

[0051] The transition metal MXene catalysts of embodiments of this invention have a general chemical formula of M.sub.yX.sub.z, wherein M is a transition metal, X is carbon and/or nitrogen (i.e., MXene), phosphor, or a chalcogen, and y and z are stoichiometric ratio integers (generally each one of 1-4, with y and z being equal or y one whole number greater than z; e.g., M.sub.2X, M.sub.3X.sub.2, and/or M.sub.4X.sub.3). In embodiments of this invention, the catalyst is or includes M.sub.n+1-X.sub.n, wherein M is a transition metal, X is carbon and/or nitrogen, and n is zero or an integer. In additional embodiments of this invention, the catalyst is or includes M.sub.xC.sub.yN.sub.z wherein M is a transition metal, C is carbon, N is nitrogen, and x, y and z are each a stoichiometric ratio integer (e.g., with each of y and z being independently one of 0 to 3, with at least one of y and z not zero, and x, y and/or z being equal or x being one whole number greater than y or z (e.g., MC, MN, M.sub.2C, M.sub.2N, M.sub.3C.sub.2, M.sub.4C.sub.3, M.sub.2CN, M.sub.3C.sub.2N, and/or M.sub.4C.sub.3N). In other embodiments of this invention, the catalyst is or includes M.sub.n−1X.sub.n, wherein M is a transition metal, X is carbon and/or nitrogen, and n is zero or an integer. Presently preferred transition metals include molybdenum, tungsten, titanium, or cobalt. Exemplary catalyst materials, without limitation, MoS.sub.2, MoSe.sub.2, Mo.sub.2C, Co.sub.2C, TiC, TiS.sub.2, TiSe.sub.2, WC, WS.sub.2, and/or WSe.sub.2.

[0052] The at least one transition metal catalyst can be provided in a variety of forms, for example, as a bulk material, in nanostructure form, as a collection of particles, and/or as a collection of supported particles. As the person of ordinary skill in the art will appreciate, the transition metal catalyst in bulk form may have a layered structure as is typical for such compounds. The transition metal catalyst may have a nanostructure morphology, including but not limited to monolayers, nanotubes, nanoparticles, nanoflakes (e.g., multilayer nanoflakes), nanosheets, nanoribbons, nanoporous solids, etc. As used herein, the term “nanostructure” refers to a material with a dimension (e.g., of a pore, a thickness, a diameter, as appropriate for the structure) in the nanometer range (i.e., greater than 1 nm and less than 1 micron).

[0053] In some embodiments, the transition metal catalyst is a layer-stacked bulk transition metal catalyst with metal atom-terminated edges. In other embodiments, transition metal catalyst nanoparticles may be used in the devices and methods of the disclosure. In other embodiments, transition metal catalyst nanoflakes may be used in the devices and methods of the disclosure. Nanoflakes can be made, for example, via liquid exfoliation, as described in Coleman, J. N. et al., “Two-dimensional nanosheets produced by liquid exfoliation of layered materials.” Science 331, 568-71 (2011) and Yasaei, P. et al., “High-Quality Black Phosphorus Atomic Layers by Liquid-Phase Exfoliation.” Adv. Mater. (2015) (doi:10.1002/adma.201405150), each of which is hereby incorporated herein by reference in its entirety. In other embodiments, transition metal catalyst nanoribbons may be used in the devices and methods of the disclosure. In other embodiments, transition metal catalyst nanosheets may be used in the devices and methods of the disclosure. The person of ordinary skill in the art can select the appropriate morphology for a particular device.

[0054] In some embodiments of the methods and devices as otherwise described herein, the transition metal catalyst nanostructures (e.g., nanoflakes, nanoparticles, nanoribbons, etc.) have an average size between about 1 nm and 1000 nm. The relevant size for a nanoparticle is its largest diameter. The relevant size for a nanoflake is its largest width along its major surface. The relevant size for a nanoribbon is its width across the ribbon. The relevant size for a nanosheet is its thickness. In some embodiments, the transition metal catalyst nanostructures have an average size between from about 1 nm to about 400 nm, or about 1 nm to about 350 nm, or about 1 nm to about 300 nm, or about 1 nm to about 250 nm, or about 1 nm to about 200 nm, or about 1 nm to about 150 nm, or about 1 nm to about 100 nm, or about 1 nm to about 80 nm, or about 1 nm to about 70 nm, or about 1 nm to about 50 nm, or 50 nm to about 400 nm, or about 50 nm to about 350 nm, or about 50 nm to about 300 nm, or about 50 nm to about 250 nm, or about 50 nm to about 200 nm, or about 50 nm to about 150 nm, or about 50 nm to about 100 nm, or about 10 nm to about 70 nm, or about 10 nm to about 80 nm, or about 10 nm to about 100 nm, or about 100 nm to about 500 nm, or about 100 nm to about 600 nm, or about 100 nm to about 700 nm, or about 100 nm to about 800 nm, or about 100 nm to about 900 nm, or about 100 nm to about 1000 nm, or about 400 nm to about 500 nm, or about 400 nm to about 600 nm, or about 400 nm to about 700 nm, or about 400 nm to about 800 nm, or about 400 nm to about 900 nm, or about 400 nm to about 1000 nm.

[0055] In certain embodiments of the methods and devices as otherwise described herein, transition metal catalyst nanoflakes have an average thickness between about 1 nm and about 100 μm (e.g., about 1 nm to about 10 μm, or about 1 nm to about 1 μm, or about 1 nm to about 1000 nm, or about 1 nm to about 400 nm, or about 1 nm to about 350 nm, or about 1 nm to about 300 nm, or about 1 nm to about 250 nm, or about 1 nm to about 200 nm, or about 1 nm to about 150 nm, or about 1 nm to about 100 nm, or about 1 nm to about 80 nm, or about 1 nm to about 70 nm, or about 1 nm to about 50 nm, or about 50 nm to about 400 nm, or about 50 nm to about 350 nm, or about 50 nm to about 300 nm, or about 50 nm to about 250 nm, or about 50 nm to about 200 nm, or about 50 nm to about 150 nm, or about 50 nm to about 100 nm, or about 10 nm to about 70 nm, or about 10 nm to about 80 nm, or about 10 nm to about 100 nm, or about 100 nm to about 500 nm, or about 100 nm to about 600 nm, or about 100 nm to about 700 nm, or about 100 nm to about 800 nm, or about 100 nm to about 900 nm, or about 100 nm to about 1000 nm, or about 400 nm to about 500 nm, or about 400 nm to about 600 nm, or about 400 nm to about 700 nm, or about 400 nm to about 800 nm, or about 400 nm to about 900 nm, or about 400 nm to about 1000 nm); and average dimensions along the major surface of about 20 nm to about 100 μm (e.g., about 20 nm to about 50 μm, or about 20 nm to about 10 μm, or about 20 nm to about 1 μm, or about 50 nm to about 100 μm, or about 50 nm to about 50 μm, or about 50 nm to about 10 μm, or about 50 nm to about 1 μm, or about 100 nm to about 100 μm, or about 100 nm to about 50 μm, or about 100 nm to about 10 μm, or about 100 nm to about 1 μm), The aspect ratio (largest major dimension:thickness) of the nanoflakes can be on average, for example, at least about 5:1, at least about 10:1 or at least about 20:1. For example, in certain embodiments the transition metal catalyst nanoflakes have an average thickness in the range of about 1 nm to about 1000 nm (e.g., about 1 nm to about 100 nm), average dimensions along the major surface of about 50 nm to about 10 μm, and an aspect ratio of at least about 5:1.

[0056] Thus, the invention provides a façade cladding system and method that provides an artificial photosynthesis process. This system has the potential to fundamentally change the way buildings are designed and constructed. With this invention, building envelope for architects and building scientists would no longer be seen as a building element through which heat is lost or gained but also a building element that actively contributes to meeting the energy needs of the building while reducing the CO.sub.2 emissions of their surrounding environment.

[0057] The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.

[0058] While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.