Patent Application
20180171492
This disclosure relates generally to photoelectrochemical cells. More particularly, the present disclosure relates to photoelectrochemical cells and methods for using such for reduction of carbon dioxide and oxidation of water.
In 2013, the global concentration of carbon dioxide in the atmosphere reached 400 parts per million (ppm) for the first time in recorded history. Such levels will cause radical and largely unpredictable changes in the environment. Recent efforts have shown that CO2 can be converted by electrochemical reduction processes driven by renewable energy sources into energy-rich fuels (e.g., syngas, methanol), offering an efficient path for both CO2 remediation and an alternative energy source.
The chemical inertness of CO2, however, renders most conversion processes highly inefficient. Current catalysts are plagued by weak binding interactions between the reaction intermediates and the catalyst (giving rise to high overpotentials), or by slow electron transfer kinetics (giving rise to low exchange current densities).
A photoelectrochemical cell capable of carbon dioxide reduction and water oxidation may generate, e.g., CO, O2, and/or H2 by irradiating a photovoltaic cell with light, generating spatially separated electron hole pairs. The generated pairs may be captured by catalysts capable of reducing carbon dioxide or oxidizing water. Current attempts at such systems have been limited by expensive light-absorbing materials and/or catalysts, and by the requirement for strongly acidic or basic reaction media, which are corrosive and difficult to manage or a large scale.
Accordingly, there remains a need for photoelectrochemical systems capable of reducing CO2 and/or oxidizing water using robust, relatively inexpensive catalysts and manageable reaction media.
One aspect of the disclosure is a method of electrochemically reducing carbon dioxide and oxidizing water in an electrochemical device, the method comprising providing an electrochemical device, the device including a first and second compartment and at least one photovoltaic cell, wherein
the first compartment includes
the second compartment includes
the at least one photovoltaic cell is in electrical contact with the anode and the cathode; and
the first compartment is in ionic contact with the second compartment; and
exposing the photovoltaic cell to light irradiation sufficient to create a potential difference between the anode and the cathode sufficient to reduce carbon dioxide at the cathode and to oxidize water at the cathode.
Another aspect of the disclosure is an electrochemical device having a first and second compartment and at least one photovoltaic cell, wherein
the first compartment includes
the second compartment includes
the at least one photovoltaic cell is in electrical contact with the anode and the cathode; and
the first compartment is in ionic contact with the second compartment.
Another aspect of the disclosure is a method of electrochemically reducing carbon dioxide in an electrochemical cell, comprising contacting the carbon dioxide with at least one transition metal dichalcogenide in the electrochemical cell and at least one helper catalyst and applying a potential to the electrochemical cell, wherein the at least one transition metal dichalcogenide is WSe2 or WS2.
Another aspect of the disclosure is a method of electrochemically reducing carbon dioxide comprising
providing an electrochemical cell having
providing carbon dioxide to the electrochemical cell; and
applying a potential to the electrochemical cell.
Another aspect of the disclosure is an electrochemical cell having a cathode in contact with at least one transition metal dichalcogenide and a first electrolyte comprising at least one helper catalyst, wherein the at least one transition metal dichalcogenide is WSe2 or WS2.
Another aspect of the disclosure is an electrochemical cell having a cathode in contact with at least one transition metal dichalcogenide and a first electrolyte comprising at least one helper catalyst, wherein the at least one transition metal dichalcogenide is WSe2 or WS2.
The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the disclosure. In this regard, no attempt is made to show structural details of the devices and methods described herein more detail than is necessary for the fundamental understanding of the devices and methods described herein, the description taken with the drawings and/or examples making apparent to those skilled in the art how the several forms of the devices and methods described herein may be embodied in practice. Thus, before the disclosed processes and devices are described, it is to be understood that the aspects described herein are not limited to specific embodiments, apparati, or configurations, and as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and, unless specifically defined herein, is not intended to be limiting.
The terms “a,” “an,” “the” and similar referents used in the context of describing the methods and devices of the disclosure (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
All methods described herein can be performed in any suitable order of steps unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein is intended merely to better illuminate the methods and devices of the disclosure and does not pose a limitation on the scope of the disclosure otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the methods and devices of the disclosure.
Unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’, ‘comprising’, 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”. Words using the singular or plural number also include the plural and singular number, respectively. Additionally, the words “herein,” “above,” and “below” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of the application.
As will be understood by one of ordinary skill in the art, each embodiment disclosed herein can comprise, consist essentially of or consist of its particular stated element, step, ingredient or component. As used herein, the transition term “comprise” or “comprises” means includes, but is not limited to, and allows for the inclusion of unspecified elements, steps, ingredients, or components, even in major amounts. The transitional phrase “consisting of” excludes any element, step, ingredient or component not specified. The transition phrase “consisting essentially of” limits the scope of the embodiment to the specified elements, steps, ingredients or components and to those that do not materially affect the embodiment.
Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by methods and devices of the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. When further clarity is required, the term “about” has the meaning reasonably ascribed to it by a person skilled in the art when used in conjunction with a stated numerical value or range, i.e. denoting somewhat more or somewhat less than the stated value or range, to within a range of ±20% of the stated value; ±19% of the stated value; ±18% of the stated value; ±17% of the stated value; ±16% of the stated value; ±15% of the stated value; ±14% of the stated value; ±13% of the stated value; ±12% of the stated value; ±11% of the stated value; ±10% of the stated value; ±9% of the stated value; ±8% of the stated value; ±7% of the stated value; ±6% of the stated value; ±5% of the stated value; ±4% of the stated value; ±3% of the stated value; ±2% of the stated value; or ±1% of the stated value.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
Groupings of alternative elements or embodiments of the disclosure disclosed herein are not to be construed as limitations. Each group member may be referred to and claimed individually or in any combination with other members of the group or other elements found herein. It is anticipated that one or more members of a group may be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.
Some embodiments of this disclosure are described herein, including the best mode known to the inventors for carrying out the devices and methods of the disclosure. Of course, variations on these described embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. Skilled artisans will employ such variations as appropriate, and the it is intended for the devices and methods of the disclosure to be practiced otherwise than specifically described herein. Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.
Furthermore, numerous references have been made to patents and printed publications throughout this specification. Each of the cited references and printed publications are individually incorporated herein by reference in their entirety.
In closing, it is to be understood that the embodiments of the disclosure disclosed herein are illustrative of the principles of the methods and devices of the present disclosure. Other modifications that may be employed are within the scope of the disclosure. Thus, by way of example, but not of limitation, alternative configurations of the methods and devices of the present disclosure may be utilized in accordance with the teachings herein. Accordingly, the methods and devices of the present disclosure are not limited to that precisely as shown and described.
In various aspects and embodiments, the disclosure relates to the electrochemical or photoelectrochemical reduction of carbon dioxide and, optionally, the oxidation of water in a device including a cathode comprising at least one transition metal dichalcogenide and, optionally, an anode comprising a metal and a photovoltaic cell. The disclosure demonstrates such methods and devices to efficiently reduce carbon dioxide, and to involve robust, relatively inexpensive catalysts and manageable reaction media.
One aspect of the disclosure is a method of electrochemically reducing carbon dioxide and oxidizing water. The method includes providing an electrochemical cell comprising a first compartment including a cathode in contact with at least one transition metal dichalcogenide, carbon dioxide, and a first electrolyte, the first compartment being in ionic contact with a second compartment including an anode in contact with at least one water oxidizing catalyst, water, and a second electrolyte, and a photovoltaic cell in electrical contact with the anode and the cathode. The method also includes exposing the photovoltaic cell to light irradiation, for example, sufficient to create a potential difference between the anode and the cathode sufficient to reduce carbon dioxide at the cathode and to oxidize water at the cathode. The light irradiation can be, for example, at an average intensity of at least 0.5 sun, for example at least 0.75 sun or at least 0.9 sun. For example, the light irradiation can be, in various example embodiments of the devices and methods as described herein, in the range of 0.5 sun to 3 sun, or 0.75 sun to 3 sun, or 0.9 sun to 3 sun, or 0.5 sun to 2 sun, or 0.75 sun to 2 sun, or 0.9 sun to 2 sun.
In the methods and devices of the disclosure, the cathode of the first compartment is in contact with at least one transition metal dichalcogenide. In some embodiments of the methods and devices as otherwise described herein, the transition metal dichalcogenide is, e.g., TiX2, VX2, CrX2, ZrX2, NbX2, MoX2, HfX2, WX2, TaX2, TcX2, or ReX2, wherein X is independently S, Se, or Te. In some embodiments, the transition metal dichalcogenide is TiX2, MoX2, or WX2, wherein X is independently S, Se, or Te. In some embodiments, the transition metal dichalcogenide is TiS2, TiSe2, MoS2, MoSe2, WS2, or WSe2. In one embodiment, the transition metal dichalcogenide is MoS2 or WS2. In another embodiment, the transition metal dichalcogenide is MoSe2 or WSe2. In yet another embodiment, the transition metal dichalcogenide is MoSe2 or WSe2. In one example, the transition metal dichalcogenide is MoS2. In another example, the transition metal dichalcogenide is WSe2.
The at least one transition metal dichalcogenide 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 dichalcogenide in bulk form may have a layered structure as is typical for such compounds. The transition metal dichalcogenide 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 μm). In some embodiments, the transition metal dichalcogenide is a layer-stacked bulk transition metal dichalcogenide with metal atom-terminated edges (e.g., MoS2 with molybdenum-terminated edges). In other embodiments, transition metal dichalcogenide nanoparticles (e.g., MoS2 nanoparticles) may be used in the devices and methods of the disclosure. In other embodiments, transition metal dichalcogenide nanoflakes (e.g., nanoflakes of MoS2) 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 dichalcogenide nanoribbons (e.g., nanoribbons of MoS2) may be used in the devices and methods of the disclosure. In other embodiments, transition metal dichalcogenide nanosheets (e.g., nanosheets of MoS2) 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.
In certain embodiments of the methods and devices as otherwise described herein, the transition metal dichalcogenide 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 dichalcogenide 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. In certain embodiments, the transition metal dichalcogenide nanostructures have an average size between from about 1 nm to about 200 nm. In certain other embodiments, the transition metal dichalcogenide nanostructures have an average size between from about 1 nm to about 400 nm. In certain other embodiments, the transition metal dichalcogenide nanostructures have an average size between from about 400 nm to about 1000 nm. In certain embodiments, the transition metal dichalcogenide nanostructures are nanoflakes having an average size between from about 1 nm to about 200 nm. In certain other embodiments, the transition metal dichalcogenide nanoflakes have an average size between from about 1 nm to about 400 nm. In certain other embodiments, the transition metal dichalcogenide nanoflakes have an average size between from about 400 nm to about 1000 nm.
In certain embodiments of the methods and devices as otherwise described herein, transition metal dichalcogenide 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 dichalcogenide 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.
In some embodiments of the methods and devices as otherwise described herein, the first electrolyte comprises at least one helper catalyst. The person of ordinary skill in the art will appreciate that the term “helper catalyst” refers to an organic molecule or mixture of organic molecules that does at least one of the following: (a) speeds up the carbon dioxide reduction reaction, or (b) lowers the overpotential of the carbon dioxide reduction reaction, without being substantially consumed in the process. The helper catalysts useful in the methods and the compositions of the disclosure are described in detail in International Application Nos. PCT/US2011/030098 (published as WO 2011/120021) and PCT/US2011/042809 (published as WO 2012/006240) and in U.S. Publication No. 2013/0157174, each of which is hereby incorporated herein by reference in its entirety. In certain embodiments, the helper catalyst is a compound comprising at least one positively charged nitrogen, sulfur, or phosphorus group (for example, a phosphonium or a quaternary amine). Aqueous solutions including one or more of: ionic liquids, deep eutectic solvents, amines, and phosphines; including specifically imidazoliums (also called imidazoniums), pyridiniums, pyrrolidiniums, phosphoniums, ammoniums, choline sulfoniums, prolinates, and methioninates can form complexes with (CO2)−, and as a result, can serve as the helper catalysts. Specific examples of helper catalysts include, but are not limited to, one or more of acetylcholines, alanines, aminoacetonitriles, methylammoniums, arginines, aspartic acids, threonines, chloroformamidiniums, thiouroniums, quinoliniums, pyrrolidinols, serinols, benzamidines, sulfamates, acetates, carbamates, inflates, and cyanides. These examples are meant for illustrative purposes only, and are not meant to limit the scope of the present disclosure. Aqueous solutions including the helper catalysts described herein can be used as the electrolyte. Such aqueous solutions can include other species, such as acids, bases and salts, to provide the desired electrochemical and physicochemical properties to the electrolyte as would be evident to the person of ordinary skill in the art.
In certain embodiments, the helper catalysts of the disclosure include, but are not limited to imidazoliums, pyridiniums, pyrrolidiniums, phosphoniums, ammoniums, sulfoniums, prolinates, and methioninates salts. The anions suitable to form salts with the cations of the helper catalysts include, but are not limited to C1-C6 alkylsulfate, tosylate, methanesulfonate, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, tetrafluoroborate, triflate, halide, carbamate, and sulfamate. In particular embodiments, the helper catalysts may be a salt of the cations selected from those in Table 1.
wherein R1-R12 are independently selected from the group consisting of hydrogen, —OH, linear aliphatic C1-C6 group, branched aliphatic C1-C6 group, cyclic aliphatic C1-C6 group, —CH2OH, —CH2CH2OH, —CH2CH2CH2OH, —CH2CHOHCH3, —CH2COH, —CH2CH2COH, and —CH2COCH3.
In certain embodiments, the helper catalyst of the methods and compositions of the disclosure is imidazolium salt of formula:
wherein R1, R2, and R3 are independently selected from the group consisting of hydrogen, linear aliphatic C1-C6 group, branched aliphatic C1-C6 group, and cyclic aliphatic C1-C6 group. In other embodiments, R2 is hydrogen, and R1 and R3 are independently selected from linear or branched C1-C4 alkyl. In particular embodiments, the helper catalyst of the disclosure is 1-ethyl-3-methylimidazolium salt. In other embodiments, the helper catalyst of the disclosure is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4).
In some embodiments, the helper catalyst may be neutral organics, such as 2-amino alcohol derivatives, isoetarine derivatives, and norepinepherine derivatives. These examples are meant for illustrative purposes only, and are not meant to limit the scope of the present disclosure.
Of course, not every substance that forms a complex with (CO2)− will act as a helper catalyst. When an intermediate binds to a catalyst, the reactivity of the intermediate decreases. If the intermediate bonds too strongly to the catalyst, the intermediate will become unreactive, so the substance will not be effective. The person of ordinary skill in the art will understand that this can provides a key limitation on substances that act as helper catalysts, and will select the helper catalyst accordingly.
In general, a person of skill in the art can determine whether a given ionic liquid is a co-catalyst for a reaction (R) catalyzed by TM DC as follows:
If V2<V1 or V2A<VIA at any concentration of the ionic liquid (e.g., between 0.0001 and 99.9999 weight %), the ionic liquid is a co-catalyst for the reaction.
The person of skill in the art will also recognize that the benefits of the helper catalyst may be realized at small amount of the helper catalyst relative to the transition metal dichalcogenide. One can obtain an estimate of the helper catalyst amount needed to change the reaction from a Pease study (“The Catalytic Combination of Ethylene and Hydrogen in the Presence of Metallic Copper III. Carbon Monoxide as a Catalyst Poison” J. Am. Chem. Soc., 1925, 47(5), pp 1235-1240), which is incorporated into this disclosure by reference in its entirety) of the effect of carbon monoxide (CO) on the rate of ethylene hydrogenation on copper. Pease found that 0.05 cc (62 micrograms) of carbon monoxide (CO) was sufficient to almost completely poison a 100 gram catalyst towards ethylene hydrogenation. This corresponds to a poison concentration of 0.0000062% by weight of CO in the catalyst. Those familiar with the technology involved here know that if 0.0000062% by weight of the poison in a catalytically active element-poison mixture could effectively suppress a reaction, then as little as 0.0000062% by weight of the helper catalyst relative to the amount of the transition metal dichalcogenide could enhance a reaction. This provides an example of a lower limit to the helper catalyst concentration relative to the transition metal dichalcogenide. Thus, in certain embodiments, the helper catalyst is present from about 0.000005 weight % to about 50 weight % relative to the weight of transition metal dichalcogenide. In some other embodiments, the amount of the helper catalyst is between about 0.000005 weight % to about 20 weight %, or about 0.000005 weight % to about 10 weight %, or about 0.000005 weight % to about 1 weight %, or about 0.000005 weight % to about 0.5 weight %, or about 0.000005 weight % to about 0.05 weight %, or about 0.00001 weight % to about 20 weight %, or about 0.00001 weight % to about 10 weight %, or about 0.00001 weight % to about 1 weight %, or about 0.00001 weight % to about 0.5 weight %, or about 0.00001 weight % to about 0.05 weight %, or about 0.0001 weight % to about 20 weight %, or about 0.0001 weight % to about 10 weight %, or about 0.0001 weight % to about 1 weight %, or about 0.0001 weight % to about 0.5 weight %, or about 0.0001 weight % to about 0.05 weight %.
Further, the helper catalyst may be dissolved in water or other aqueous solution, a solvent for the reaction, an electrolyte, an acidic electrolyte, a buffer solution, an ionic liquid, an additive to a component of the system, or a solution that is bound to at least one of the catalysts in a system. These examples are meant for illustrative purposes only, and are not meant to limit the scope of the present disclosure.
In some embodiments of the methods and devices as otherwise described herein, the first electrolyte is an aqueous solution. In certain embodiments, the first electrolyte is an aqueous solution comprising the at least one helper catalyst. In some embodiments, the helper catalyst is present in the aqueous solution in a concentration within the range of about 5 vol. % to about 75 vol. %, e.g., about 10 vol. % to about 75 vol. %, or about 15 vol. % to about 75 vol. %, or about 20 vol. % to about 75 vol. %, or about 25 vol. % to about 75 vol. %, or about 30 vol. % to about 75 vol. %, or about 35 vol. % to about 75 vol. %, or about 40 vol. % to about 75 vol. %, or about 45 vol. % to about 75 vol. %, or about 30 vol. % to about 70 vol. %, or about 35 vol. % to about 65 vol. %, or about 40 vol. % to about 60 vol. %, or about 45 vol. % to about 55 vol. %, or about 5 vol. %, or about 10 vol. %, or about 15 vol. %, or about 20 vol. %, or about 25 vol. %, or about 30 vol. %, or about 35 vol. %, or about 40 vol. %, or about 45 vol. %, or about 50 vol. %, or about 55 vol. %, or about 60 vol. %, or about 65 vol. %, or about 70 vol. %, or about 75 vol. %.
The person of ordinary skill in the art will appreciate that the first electrolyte of the methods and devices as otherwise described herein may further include, e.g., nonaqueous solvents, a buffer solution, an additive to a component of the system, or a solution that is bound to a catalyst included in the first compartment. In certain embodiments of the methods and devices as otherwise described herein, the first electrolyte may further comprise other species, such as acids, bases, and salts. The inclusion of such other species would be evident to the person of ordinary skill in the art depending on the desired electrochemical and physicochemical properties of the first electrolyte, and are not meant to limit the scope of the present disclosure.
The devices and methods of the disclosure involve reducing CO2. The person of ordinary skill in the art will appreciate that, e.g., in water, CO2 may form chemical derivatives such as carbonic acid, bicarbonate, or carbonate. As used herein, CO2 and such derivatives may be referred to interchangeably, i.e., reference to CO2 reduction in an aqueous solution may also refer to carbonate, bicarbonate, or carbonic acid reduction in an aqueous solution. In some embodiments of the methods and devices as otherwise described herein, a reactant comprising CO2, carbonate, or bicarbonate is fed into the first compartment of the electrochemical device. For example, gaseous CO2 may be continuously bubbled through the first compartment. A voltage is applied to the first compartment, i.e., upon exposure of the photovoltaic cell to light irradiation, and the CO2 reacts to form new chemical compounds. As the person of ordinary skill in the art will recognize, CO2 (as well as carbonate or bicarbonate) may be reduced into various useful chemical products, including but not limited to CO, syngas (mixture of CO and H2), OH−, HCO−, H2CO, (HCO2)−, H2CO2, CH3OH, CH4, C2H4, CH3CH2OH, CH3CO−, CH3COOH, C2H6, O2, H2, (COOH)2, and (COO−)2. In certain embodiments, CO2 may be reduced to form CO, H2, or a mixture of CO and H2.
Advantageously, the carbon dioxide used in the embodiments of the disclosure can be obtained from any source, e.g., an exhaust stream from fossil-fuel burning power or industrial plants, from geothermal or natural gas wells or the atmosphere itself. In certain embodiments, carbon dioxide is anaerobic. In other embodiments, carbon dioxide is obtained from concentrated point sources of its generation prior to its release into the atmosphere. For example, high concentration carbon dioxide sources are those frequently accompanying natural gas in amounts of 5 to 50%, those from flue gases of fossil fuel (coal, natural gas, oil, etc.) burning power plants, and nearly pure CO2 exhaust of cement factories and from fermenters used for industrial fermentation of ethanol. Certain geothermal steams also contain significant amounts of CO2. In other words, CO2 emissions from varied industries, including geothermal wells, can be captured on-site. Separation of CO2 from such exhausts is well-known. Thus, the capture and use of existing atmospheric CO2 in accordance with embodiments of the disclosure allows CO2 to be a renewable and unlimited source of carbon.
In some embodiments of the methods and devices as otherwise described herein, the reduction of carbon dioxide may be initiated at high current densities. For example, in certain embodiments, the current density of carbon dioxide reduction is at least 30 mA/cm2, or at least 40 mA/cm2, or at least 50 mA/cm2, or at least 55 mA/cm2, or at least 60 mA/cm2, or at least 65 mA/cm2. In one embodiment, the current density of carbon dioxide reduction is between about 30 mA/cm2 and about 130 mA/cm2, or about 30 mA/cm2 and about 100 mA/cm2, or about 30 mA/cm2 and about 80 mA/cm2, or about 40 mA/cm2 and about 130 mA/cm2, or about 40 mA/cm2 and about 100 mA/cm2,or about 40 mA/cm2 and about 80 mA/cm2, or about 50 mA/cm2 and about 70 mA/cm2, or about 60 mA/cm2 and about 70 mA/cm2, or about 63 mA/cm2 and about 67 mA/cm2, or about 60 mA/cm2, or about 65 mA/cm2, or about 70 mA/cm2.
In some embodiments of the methods and devices as otherwise described herein, the reduction of carbon dioxide may be initiated at low overpotential. For example, in certain embodiments, the initiation overpotential is less than about 200 mV. In other embodiments, the initiation overpotential is less than about 100 mV, or less than about 90 mV, or less than about 80 mV, or less than about 75 mV, or less than about 70 mV, or less than about 65 mV, or less than about 60 mV, or less than about 57 mV, or less than about 55 mV, or the initiation overpotential is within the range of about 50 mV to about 100 mV, or about 50 mV to about 90 mV, or about 50 mV to about 80 mV, or about 50 mV to about 75 mV, or about 50 mV to about 70 mV, or about 50 mV to about 65 mV, or about 50 mV to about 60 mV. In some embodiments, the reduction of carbon dioxide is initiated at overpotential of about 50 mV to about 57 mV, or about 51 mV to about 57 mV, or about 52 mV to about 57 mV, or about 52 mV to about 55 mV, or about 53 mV to about 55 mV, or about 53 mV, or about 54 mV, or about 55 mV.
The methods described herein can be performed at a variety of pressures and temperatures, and a person of skill in the art would be able to optimize these conditions to achieve the desired performance. For example, in certain embodiments, the methods of the disclosure are performed at a pressure in the range of about 0.1 atm to about 2 atm, or about 0.2 atm to about 2 atm, or about 0.5 atm to about 2 atm, or about 0.5 atm to about 1.5 atm, or about 0.8 atm to about 2 atm, or about 0.9 atm to about 2 atm, about 0.1 atm to about 1 atm, or about 0.2 atm to about 1 atm, or about 0.3 atm to about 1 atm, or about 0.4 atm to about 1 atm, or about 0.5 atm to about 1 atm, or about 0.6 atm to about 1 atm, or about 0.7 atm to about 1 atm, or about 0.8 atm to about 1 atm, or about 1 atm to about 1.5 atm, or about 1 atm to about 2 atm. In one particular embodiment, the methods of the disclosure are carried at a pressure of about 1 atm. In other embodiments, the methods of the disclosure are carried out at a temperature within the range of about 0° C. to about 50° C., or of about 10° C. to about 50° C., or of about 10° C. to about 40° C., or of about 15° C. to about 35° C., or of about 20° C. to about 30° C., or of about 20° C. to about 25° C., or at about 20° C., or at about 21° C., or at about 22° C., or at about 23° C., or at about 24° C., or at about 25° C. In one particular embodiment, the methods of the disclosure are carried out at a temperature of about 20° C. to about 25° C. The methods of the disclosure may last, for example, for a time within the range of about several minutes to several days and months.
Advantageously, in certain embodiments the methods described herein can be operated at a Faradaic efficiency (FE) within the range of 0 to 100% for the reduction of carbon dioxide to CO. In some embodiments, the Faradaic efficiency of the carbon dioxide-to-CO reduction is at least about 3%, or at least about 5%, or at least about 8%, or at least about 10%, or at least about 20%, or at least about 25%, or at least about 50%, or at least about 60%, or at least about 70%, or at least about 75%, or at least about 80%, or at least about 85%.
The person of ordinary skill in the art will appreciate that the cathode of the first compartment may comprise any of a number of conductive materials known in the art. In some embodiments, the cathode comprises, e.g., copper, aluminum, carbon black, or stainless steel. In some embodiments, the cathode comprises stainless steel. The person of ordinary skill in the art will further appreciate that the at least one transition metal dichalcogenide may be contacted with the cathode by a variety of means. For example, in some embodiments, the transition metal dichalcogenide may be disposed on the cathode. In some embodiments, the transition metal dichalcogenide is disposed on a porous member. The porous member may be electrically conductive, in which case the porous member may be in electrical contact with the cathode. In cases where the porous member is not electrically conductive, the person of ordinary skill in the art can arrange for the electrical connection of the cathode to be made to some other part of the at least one transition metal dichalcogenide.
In some embodiments of the methods as otherwise described herein, the at least one transition metal dichalcogenide is coated onto the cathode at a thickness of, e.g., up to 1000 μm. The person of ordinary skill in the art will appreciate that the thickness of the at least one transition metal dichalcogenide may be any convenient thickness, provided CO2 can be reduced in the electrochemical device.
In the methods and devices of the disclosure, the second compartment includes an anode in contact with at least one water oxidizing catalyst. As used herein, the term “water oxidizing catalyst” refers to a compound capable of catalyzing the reaction:
2H2O→O2+4H+,
and may be used interchangeably with “oxygen evolving catalyst.” In some embodiments of the methods and devices as otherwise described herein, the water oxidizing catalyst comprises cobalt, e.g., Co3+.
In the methods and devices of the disclosure, the second compartment includes water and a second electrolyte. In some embodiments of the methods and devices as otherwise described herein, the second electrolyte and the water comprise an aqueous solution. In some embodiments, the aqueous solution comprises phosphate. In some embodiments, the phosphate comprises potassium phosphate, e.g., KH2PO4. In some embodiments, the phosphate is present in the aqueous solution in a concentration within the range of about 0.01 mM to about 100 mM, e.g., about 0.05 mM to about 50 mM, or about 0.05 mM to about 10 mM, or about 0.05 mM to about 5 mM, or about 0.05 mM to about 1 mM, or about 0.05 mM to about 0.9 mM, or about 0.05 mM to about 0.8 mM, or about 0.05 mM to about 0.7 mM, or about 0.05 mM to about 0.6 mM, or about 0.05 mM to about 0.5 mM, or about 0.05 mM to about 0.45 mM, or about 0.1 mM to about 0.4 mM, or about 0.15 mM to about 0.35 mM, or about 0.2 mM to about 0.3 mM.
The person of ordinary skill in the art will appreciate that the second electrolyte of the methods and devices as otherwise described herein may further include, e.g., nonaqueous solvents, a buffer solution, an additive to a component of the system, or a solution that is bound to a catalyst included in the second compartment. In certain embodiments of the methods and devices as otherwise described herein, the second electrolyte may further comprise other species, such as acids, bases, and salts. The inclusion of such other species would be evident to the person of ordinary skill in the art depending on the desired electrochemical and physicochemical properties of the second electrolyte, and are not meant to limit the scope of the present disclosure.
The person of ordinary skill in the art will appreciate that the anode of the second compartment may comprise any of a number of conductive materials known in the art. In some embodiments, the anode comprises, e.g., copper, aluminum, carbon black, or stainless steel. In some embodiments, the anode comprises indium tin oxide (ITO). The person of ordinary skill in the art will further appreciate that the at least one water oxidation catalyst may be contacted with the anode by a variety of means. For example, in some embodiments, the water oxidation catalyst may be disposed on the cathode. In some embodiments, the water oxidation catalyst is disposed on a porous member. The porous member may be electrically conductive, in which case the porous member may be in electrical contact with the anode. In cases where the porous member is not electrically conductive, the person of ordinary skill in the art can arrange for the electrical connection of the anode to be made to some other part of the at least one water oxidation catalyst.
In some embodiments of the methods as otherwise described herein, the at least one water oxidation catalyst is coated onto the cathode at a thickness of, e.g., up to 1000 μm. The person of ordinary skill in the art will appreciate that the thickness of the at least one water oxidation catalyst may be any convenient thickness, provided water can be oxidized in the electrochemical device.
In the methods and devices of the disclosure, the electrochemical device includes at least one photovoltaic cell. The person of ordinary skill in the art will appreciate that the photovoltaic cell may provide the electrical energy for the electrochemical reduction of carbon dioxide and the oxidation of water. The person of ordinary skill in the art will appreciate that the photovoltaic cell may be any of a variety of types and/or arrangements of photovoltaic cells, provided the potential supplied to the cathode and anode is sufficient to drive carbon dioxide reduction and water oxidation in the electrochemical device.
In some embodiments of the methods and devices as otherwise described herein, the at least one photovoltaic cell is a multi-junction photovoltaic cell. In some embodiments, the electrochemical device includes two or more photovoltaic cells connected in series. For example, in certain embodiments, the electrochemical device comprises two multi-junction photovoltaic cells connected in series (See, e.g.,
In some embodiments of the methods and devices as otherwise described herein, the at least one photovoltaic cell comprises Si or Ge. In some embodiments, the at least one photovoltaic cell comprises a layer comprising amorphous Si. In some embodiments, the at least one photovoltaic cell comprises a layer comprising amorphous SiGe. In some embodiments, the at least one photovoltaic cell is a multi-junction cell photovoltaic cell comprising one or more layers comprising amorphous Si, and one or more layers comprising amorphous SiGe. For example, in certain embodiments, the electrochemical device includes two identical multi-junction photovoltaic cells connected in series, each cell comprising one layer comprising amorphous Si and two layers comprising amorphous SiGe.
In some embodiments of the methods and devices as otherwise described herein, the at least one photovoltaic cell is capable of providing at least about 2.5 V across the cell, e.g., at least about 2.6 V, or at least about 2.7 V, or at least about 2.8 V, or at least about 2.9 V, or at least about 3 V. In some embodiments, the at least one photovoltaic cell is capable of operating with an efficiency of at least about 4%, e.g. at least about 4.5%, or at least about 5%, or at least about 5.5%, or at least about 6%.
In the methods and devices of the disclosure, the first compartment and the second compartment are in ionic contact. As used herein, the term “ionic contact” refers to the ability to transport ions from one area to a second area. For example, ions may be transported from a first compartment to a second compartment in ionic contact therewith. Ionic contact may be selective for ions, e.g., by limiting transport of neutral molecules. Ionic contact may be selective for a specific charge, e.g., selective for positive ions, or for a specific ion, e.g., selective for protons. The person of ordinary skill in the art will appreciate that there exists in the art a variety of means for providing ionic contact, e.g., between two compartments, such as, for example, ion-conductive membranes, proton-conductive membranes, etc.
In some embodiments of the methods and devices of the disclosure, the first and second compartments are in general contained and physically separated, e.g., by glass, steel, indium tin oxide, etc., but in part are separated by an ion-conductive (i.e., ion-exchange) membrane, e.g., a proton-conductive membrane. In some embodiments, the first and second compartments are physically contained, e.g., by glass, steel, indium tin oxide, etc., but are separated only by an ion-conductive membrane, e.g., a proton-conductive membrane. The person of ordinary skill in the art will appreciate that the area of ionic contact (e.g., the area of a proton-conductive membrane) may be optimized to achieve a desired effect in the electrochemical device.
In some embodiments of the methods and devices as otherwise described herein, the first and second compartments are in ionic contact through a proton-conductive membrane. In some embodiments, the proton-conductive membrane is a polymer electrolyte (i.e., an ionomer) membrane comprising, e.g., a sulfonated fluoropolymer. In some embodiments, the first and second compartments are in ionic contact through a tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (i.e., Nafion).
The person of ordinary skill in the art will appreciate that the compartments and at least one photovoltaic cell of the electrochemical device may be arranged in a variety of different ways such that the at least one photovoltaic cell is in electrical contact with the anode and cathode and the first and second compartments are in ionic contact. In some embodiments, the first and second compartments are in general physically separated by the photovoltaic cell and, in part separated by an ion-conductive membrane, e.g., a proton-conductive membrane, wherein the anode and cathode are disposed on and in electrical contact with the photovoltaic cell. One such embodiment is shown in schematic view in
In some embodiments, the first and second compartments may be separated only by an ion-conductive membrane, e.g., a proton-conductive membrane, wherein the cathode and anode of the first and second compartments, respectively, are electrically connected to the at least one photovoltaic cell by conductive wires. One such embodiment is shown in schematic view in
Another aspect of the disclosure is an electrochemical device having a first and second compartment at least one photovoltaic cell, wherein the first compartment includes a cathode in electrical contact with the at least one transition metal dichalcogenide, a first electrolyte, and carbon dioxide, carbonic acid, or a carbonic acid salt; the second compartment includes an anode in electrical contact with at least one water oxidizing catalyst, a second electrolyte, and water; and wherein the at least one photovoltaic cell is in electrical contact with the anode and the cathode, and the first compartment is in ionic contact with the second compartment.
In some embodiments of the electrochemical device, the first compartment, cathode, transition metal dichalcogenide, first electrolyte, second compartment, anode, water oxidizing catalyst, second electrolyte, and photovoltaic cell are as otherwise described herein. In some embodiments of the electrochemical device as otherwise described herein, the first electrolyte comprises at least one helper catalyst. In some embodiments, the at least one transition metal dichalcogenide is MoS2 or WSe2. In some embodiments, the at least one transition metal dichalcogenide is in nanoflake, nanosheet, or nanoribbon form. In some embodiments, the helper catalyst is 1-ethyl-3-methylimidazolium tetrafluoroborate. In some embodiments, the first electrolyte is an aqueous solution. In some embodiments, the helper catalyst is present in the aqueous solution in a concentration within the range of about 25 vol. % to about 75 vol. %. In some embodiments, the electrochemical device as otherwise described herein is for use in reducing carbon dioxide and oxidizing water.
Another aspect of the disclosure is a method of electrochemically reducing carbon dioxide in an electrochemical cell, comprising contacting the carbon dioxide with at least one transition metal dichalcogenide in the electrochemical cell and at least one helper catalyst and applying a potential to the electrochemical cell, wherein the at least one transition metal dichalcogenide is WSe2 or WS2. In some embodiments of the method, the helper catalyst is as otherwise described herein. In some embodiments, the electrochemical cell comprises a cathode as otherwise described herein, wherein the cathode is in contact with the at least one transition metal dichalcogenide. In some embodiments, the electrochemical cell comprises a first electrolyte as otherwise described herein, wherein the first electrolyte comprises the at least one helper catalyst.
Another aspect of the disclosure is a method of electrochemically reducing carbon dioxide comprising providing an electrochemical cell having a cathode in contact with at least one transition metal dichalcogenide, and a first electrolyte comprising at least one helper catalyst in contact with the cathode and the at least one transition metal dichalcogenide, wherein the at least one transition metal dichalcogenide is WSe2 or WS2; providing carbon dioxide to the electrochemical cell; and applying a potential to the electrochemical cell. In some embodiments, the cathode, first electrolyte, and helper catalyst are as otherwise described herein. In some embodiments, the transition metal dichalcogenide is in bulk form. In some embodiments, the transition metal dichalcogenide is in nanoparticle form, as otherwise described herein. In some embodiments, the transition metal dichalcogenide nanoparticles have an average size between about 1 nm and 400 nm. In some embodiments, the transition metal dichalcogenide is in nanoflake, nanosheet, or nanoribbon form, as otherwise described herein. In some embodiments, the transition metal dichalcogenide nanoflakes, nanosheets, or nanoribbons have an average size between about 1 nm and 400 nm. In some embodiments, the helper catalyst is as otherwise described herein. In some embodiments, the helper catalyst is 1-ethyl-3-methylimidazolium tetrafluoroborate.
Another aspect of the disclosure is an electrochemical cell having a cathode in contact with at least one transition metal dichalcogenide and a first electrolyte comprising at least one helper catalyst, wherein the at least one transition metal dichalcogenide is WSe2 or WS2. In some embodiments of the cell, the cathode, first electrolyte, and cathode are as otherwise described herein. In some embodiments, the first electrolyte is an aqueous solution of the helper catalyst. In some embodiments, the helper catalyst is 1-ethyl-3-methylimidazolium tetrafluoroborate. In some embodiments the cell as otherwise described herein, the cell is for use in reducing carbon dioxide.
The Examples that follow are illustrative of specific embodiments of the disclosure, and various uses thereof. They are set forth for explanatory purposes only, and are not to be taken as limiting the disclosure.
Transition metal dichalcogenides (e.g., MoS2, MoSe2, WS2, and WSe2) were synthesized through direct reaction of pure forms of the relevant elements followed by a vapor transport process in an evacuated ampule at elevated temperatures. In this method, powders of the transition metals and chalcogens (>99.99% trace metal basis purity) were mixed in the desired stoichiometric ratio and loaded into a quartz ampule. The total loaded weight was about one gram. Each quartz ampule had a 1 cm internal diameter and a 20 cm length. The ampule was then evacuated with a turbo molecule pump (<10−6 mbar) and sealed with a hydrogen torch. The ampule was placed into a two-zone CVD furnace and the temperature of both zones was raised to 1080° C. over 1 day. The temperature of the empty end of the ampule (the cold zone) was then gradually cooled to 950° C. over 4 days, while the other end was maintained at 1080° C., providing single crystalline grains with pristine structure via direct vapor transport. The system was slowly cooled to room temperature over 1 day, after which the crystalline material (See, e.g.,
The crystalline grains produced according to Example 1 were ground to a powder. Nanoflakes were formed by sonicating a dispersion of 300 mg of ground transition metal dichalcogenide powder in 60 mL of isopropanol. The dispersion was sonicated for 30 hours, using a sonication probe (Vibra Cell Sonics 130 W). The sonicated dispersions were then centrifuged for 60 minutes at 2000 rpm, after which the supernatant (the top two thirds of the centrifuged dispersion) was collected by pipette and stored in a glass vial.
Dynamic light scattering (DLS) experiments were performed to measure nanoflake sizes using a NiComp ZLS 380 system configured with a 35 mW semiconductor laser (670 nm emission) and a thermoelectric temperature control for samples (held at 25° C.). Nanoflakes dispersed in isopropanol were measured, providing the normal distributions shown in
The nanoflakes were also characterized by Raman spectroscopy, using a HORIBA LabRAM HR Evolution confocal Raman microscope configured with a 532 nm laser source, a 1200 g/mm grating, a Horiba Andor detector, and a 100x objective. Laser powers at the sample were held between 1-15 mW. Integration times and averaging parameters were chosen to maximize the signal-to-noise ratio. Results are shown in
Finally, WSe2 nanoflakes were imaged with scanning electron microscopy (SEM) to understand the microscale morphology of the nanoflakes. Samples were imaged with a Carl Zeiss SEM instrument integrated in a Rait e-LiNE plus ultra-high resolution electron beam lithography system. Samples were kept at a distance of 10 mm from the electron source, held at 10 kV. Results are shown in
A two-compartment three-electrode electrochemical cell was used to perform CO2 reduction reactions (See,
Potential vs. RHE=Applied Potential vs. Ag/AgCl+0.197 V+(0.0592×pH)
The cathode and anode were separated by an ion-conductive membrane to eliminate potential product oxidation at the anode surface. All experiments were performed using a rotating disk electrode (RDE) submerged in the cell. To eliminate any effect of mass transport during the reactions, the working electrode was rotated at 1000 rpm. The cell was connected to a potentiostat (CH Instruments) connected to a computer through CH Instruments software. A 6 mm polyethylene tube bubbled CO2 (99.9% UHP, Praxair) through the electrolyte solution for 30 minutes prior to experiments. Results of cyclic voltammetry and chronoamperometry experiments, shown in
Table 2, below, shows the pH of the aqueous electrolyte solution of the three-electrode cell as a function of different concentrations of EMIM-BF4.
A working electrode coated with WSe2 nanoflakes was tested at a potential of −0.764 V vs. RHE in a chronoamperometry experiment carried out according to Example 4, at various pHs (i.e., different concentration of EMIM-BF4). The results, shown in
To further characterize the catalytic activity of the materials of Examples 1-5, a roughness factor (RF) technique was employed to determine the number of active edge sites of the materials. All experiments were performed using the same surface area, (the catalyst loadings for each material were different, however). The RF number of WSe2 nanoflakes were estimated by comparing its double layer capacitance (Ddl) with a flat standard capacitor of MoS2 (See, Table 3). Cyclic voltammetry experiments were performed at different scan rates in 0.5M H2SO4 to calculate the Cdl of each material (See,
Density of active sites(sites/cm2)=(Density of active sites of standard sample)×RF
Additionally, the CO formation TOF of active sites for CO2 reduction by WSe2 nanoflakes, bulk MoS2, and Ag nanoparticles in the aqueous EMIM-BF4 electrolyte was calculated at various overpotentials using the following equation:
CO formation TOF(s−1)=i0(A/cm2)×CO formation FE/{[active site density(sites/cm2)]×[1.602×10−19(C/e−)×[2e−/CO2]}
Results, provided in
The products of the electrochemical experiments of Example 4 were analyzed with gas chromatography (GC) using an SRI 8610C GC system equipped with a 72 in.×18 in. stainless steel column packed with molecular sieves. A thermal conductivity detector was used to analyze and differentiate the injected samples. Ultra-high-purity helium and nitrogen (Praxair) were used as carrier gases for CO and H2 detection, respectively.
The GC apparatus was calibrated to determine the moles of products (i.e., CO and H2) using 2, 5, 10 and 20 vol % of CO and H2 in He and N (99.99% research grade, Praxair), respectively. The known volume of standard samples (1 mL) were injected at a constant pressure (10 psi) and temperature (25° C.), using helium and nitrogen as carrier gases for CO and H2 detection, respectively. A distinct CO peak was apparent at 3.83 min., and H2 was detected at 0.98 min. Calibration curves (See,
1 mL of the gas products of the electrochemical experiments of Example 4, carried out for a desired time (e.g., 10 min.), was injected into the GC instrument using a sample lock syringe. Only CO and H2 were detected.
In order to identify any other potential carbon-based products, 13CO2 was reduced in the electrochemical experiment of Example 4, the products of which were analyzed using differential electrochemical mass spectrometry (DEMS) with a quadrupole detector (HPR-20, purchased from Hiden Analytical Inc.). The DEMS instrument was operated at ultra-high-vacuum pressure (1×10−6 torr) through the analysis. The product stream was injected to the DEMS instrument at a flow rate of 0.8-1 mL/min using a quartz coated very-low-flow capillary line. Analysis of the product stream, shown in
The Faradaic efficiency (FE) of the transition metal dichalcogenides materials produced according to Example 2 were calculated using the following equation:
FE=100×(moles of product)/[{j(mA/cm2)×t(s)}/nF ]
wherein the number of moles of product is determined according to Example 7, j is the curve area of the plot of current density vs. time, provided in Example 4, n is the number of electrons required for the reduction of CO2 to CO (i.e., 2), and F is the Faradic number.
The resultant FE values (See,
The product of the current density and FE of the materials of the preceding Examples was plotted as a function of overpotential to provide an overview of catalytic performance. Results, provided in
The long-term stability of WSe2 nanoflakes were investigated in an electrochemical experiment configured and carried out similarly to Example 4. A magnetic stirrer was placed in the electrolyte solution to eliminate any potential complications due to mass transport. The stability of the WSe2-coated electrode was recorded for 27 hours at a potential of −0.364 V (0.254 V overpotential) using a Voltalab PGZ100 potentiostat (Radiometer Analytical SAS) calibrated with a RCB200 resistor capacitor box. The potentiostat was connected to a PC using Volta Master (Version 4) software. The results, shown in
All chemicals were used as received, without any purification, unless required. Cobalt nitrate hexahydrate (Alfa Aesar), potassium based buffer solution (0.071 M KPi, pH=7, Sigma-Aldrich), Nafion 117 (10.0 cm×10.0 cm, FuelCellsEtc) were used in the following configuration. Triple-junction amorphous-Si solar cells were purchased from Xun-light Corp. (Toledo, Ohio). The acrylic used for the chambers was purchased from Total Plastics Inc.
A photoelectrochemical chamber was machined from acrylic plastic and assembled with acrylic glue. The transparent chamber was separated into two compartments by two tandem amorphous-Si-based triple-junction (a-Si/A-SiGe/A-SiGe) photovoltaic (PV) cell comprising an indium tin oxide (ITO) anode layer disposed on the exposed a-Si layer and a stainless steel cathode layer disposed on the exposed a-SiGe layer, connected in series through copper tape and separated by a piece of nafion membrane (See,
A cobalt oxygen-evolving catalyst was electrodeposited onto the ITO surface of the PV cells from a cobalt (II) nitrate hexahydrate solution. The electrodeposition was carried out using a solution prepared by mixing 73 mg of cobalt nitrate hexahydrate in 500 mL of potassium phosphate (2.6×10−4 M K+, pH=7) using a three-electrode cell configuration comprising a platinum mesh counter electrode and a Ag/AgCl reference electrode, wherein the ITO layer of the PV cell served as the working electrode. Electrodeposition was carried out at a potential of 1.5 V vs Ag/AgCl for 5 minutes, without stirring and without any i-R compensation. The stainless steel layer was covered throughout the electrodeposition.
WSe2 nanoflakes were prepared according to Example 2 and suspended in isopropanol. The suspension was drop cast onto the stainless steel anode of the PV cells, which were allowed to dry completely.
The catalyst-coated anode/PV cell/cathode unit and a section of nafion membrane (activated by treatment with 5 wt. % KOH) were configured to separate the two compartments of the device while allowing for ionic contact between the compartments (See,
Upon exposure to light irradiation, the photoelectrochemical device configured according to Example 10 operates first in a transient regime, after which the device operates in a “steady-state.” Initially, the H+ concentration in the solution of the anodic compartment is much lower than that of the cathodic compartment. Upon exposure to light irradiation, H+ is produced at the anode (i.e., through water oxidation), lowering the pH of the anodic solution, while the reduction of CO2 at the cathode consumes available H+, increasing the pH of the cathodic solution. During this time, K+ ions diffuse through the proton-conductive membrane to compensate for charge imbalance. After approximately 5 minutes, the pH of the solutions of both compartments equilibrate at 3.35, at which point the electrochemical device reaches steady-state operation, wherein diffusion of H+ from the anodic compartment to the cathodic compartment overtakes that of K+ (See,
K+ crossover in the device was quantified using a PerkinElmer Inductively Coupled Plasma—Optical Emission Spectroscopy (ICP-OES, Optima 5300DV) instrument. Solution samples were collected at various time intervals and diluted by a factor of 5 or 20 using 2% HNO3, based on sample volume. Diluted samples were analyzed using an ESI Fast auto-sampler coupled with the ICP-OES device. Measurements demonstrated that the K+ concentration of the aqueous solution of EMIM-BF4 (i.e., the cathodic solution) reached 1.43×10−4 M after five minutes of exposure of the photovoltaic cell to irradiation, after which the concentration remained constant. This result is consistent with the slightly increased pH of the cathodic solution after the same period of operation, which corresponds to a change in H+ concentration of 1.52×10−4 M. The performance of the device decreases after about 5 hours of continuous operation.
The product stream of the device operated according to Example 11 was analyzed according to the method of Example 7. Results, shown in
As evidenced by
The volume of water produced through the device operation of Example 11 was calculated. Because water and CO are be produced in a stoichiometric ratio of 1:1 at the cathode of the device, the rate of water production, under 1 sun of illumination is known to be 2.75×10−7 mol/s, or 9.9×10−4 mole of water generated over 1 hour of operation. After 100 hours of operation, only 1.8 mL water will have been produced, which will have a negligible effect on the pH or composition of the cathodic solution. Table 5 shows the amount of produced water at different levels of illumination of the device configured according to Example 10.
The solar to fuel conversion efficiency (SFE) of the device operation of Example 11 was calculated using the following equation:
wherein N1 and N2 are the molar quantities of produced gas per unit time (mol/s) provided by the GC analysis of Example 12, E1 and E2 are the energy densities of the corresponding gas (kJ/mol), which are 283.24 and 140 kJ/mol for CO and H2, respectively, Acat is the overall catalystic surface area available for the reaction (cm2), which is 18 cm2, and Ug is the total solar irradiance (mW/cm2), which is 100 mW/cm2 for 1 sun of illumination.
To estimate the uncertainty of the calculations, the partial derivative method is used to calculate the sensitivity of the SFE values to different input parameters. For this purpose, each parameter of the above equation was perturbed by a small amount (∂xi) around its typical value (xi) to provide a corresponding change in the extracted SFE (∂η). The dimensionless sensitivities were then calculated using the following equation:
The overall uncertainty (uη) was also calculated, using the following equation:
where ux
for N1 and N2 was calculated based on the standard deviations of the values from three different experiments, which were 0.07 and 0.09, respectively. Because E1 and E2 values were literature values,
for these values were considered to be zero. The value of
for Ug was based on the fluctuation in the response of the photodiode during device operation. The uncertainty in Acat was 0.05 (based on measurement with a Vernier caliper). A summary of the uncertainty analysis is provided in Table 6. The error bars shown in
The SFE values over 5 hours and 100 hours of continuous operation according to Example 11 were also calculated, and are provided in
The solar-to-electricity conversion efficiency of a triple-junction photovoltaic cell comprising an ITO layer and a cobalt oxygen evolving catalyst disposed thereon was measured under one sun of illumination. The voltage was measured directly with a multi-meter while the resistance of the circuit was changed using variable resistors. The open circuit voltage (VOC), short circuit current, and average fill factor of the cell were 2.12 V, 6.1 mA/cm2 and 0.55, respectively. Accordingly, a dry cell efficiency of 7.1% was calculated by dividing the product of the three aforementioned parameters by the energy of one sun of illumination (100 mW/cm2). The VOC of two such cells connected in series was 3.6 V, which decreased to 3V when submerged in the electrolyte solutions of Example 10 (the short circuit current remained constant). The fill factor was assumed to remain relatively constant under the configuration and operation parameters of Examples 10 and 11. Accordingly, the efficiency of the PV cell was estimated to be about 6% when submerged in the electrolyte.
Electrochemical impedance spectroscopy (EIS) experiments were performed using an electrochemical cell and electrodes configured similarly to Example 4. The Nyquist plot for different CO2 reduction over-potentials, e.g., 150, 200, 300, 400, and 500 mV, were recorded at a small (10 mV) AC voltage amplitude (to avoid nonlinearity) and over a frequency range of 10 to 105 Hz, using a Voltalab PGZ100 potentiostat. An equivalent Randles circuit model was fit to the data to calculate Rct for each catalyst system.
The work function for four transition metal dichalcogenides and Ag nanoparticles was measured by ultraviolet photoelectron spectroscopy (UPS) (See,
Scanning transmission electron microscopy (STEM) images were acquired on a JEAL JEM-ARM200CF instrument operated at 200 kV. Images were acquired in either high/low angle annular dark field (H/LAADF) or annular bright field (ABF) mode.
X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Scientific ESCALAB 250Xi instrument equipped with an electron flood and scanning ion gun. Spectra were calibrated to the C1s binding energy of 284.8 eV. As shown in
Density functional theory (DFT) calculations were carried out to investigate the catalytic properties of transition metal dichalcogenide nanoflakes (e.g., nanoflakes prepared according to Example 2). Periodic DFT calculations were performed with plane wave basis sets in the VASP package. Reaction free energies and density of states (DOS) calculations were performed on single-layer nanoribbons of the transition metal dichalcogenides truncated with zig-zag edges.
Both monolayer slabs and nanoribbons of the transition metal dichalcogenides were used to calculate the work functions. For the nanoribbons, each unit well included 4×4 (16 total) metal atoms and 32 S or Se atoms (for low CO coverage calculations, the unit cell included 6×4 metal atoms and 48 S or Se atoms), containing both the metal and the S/Se edges. A 10 A vacuum space was set both on top of the metal edge and between two nanoribbon periodic images. For the single-layer slabs for work function calculations, only the minimum atoms to construct a unit cell were used. The Ag(111) surface was constructed by a 4×4×4 slab in a unit cell, with 10 Å vacuum space. A kinetic energy cutoff of 400 eV was used for all calculations. All atoms in the system were allowed to relax, while the cell shape and volume were kept fixed. K-point grids of 3×1×1 and 3×3×1 were used for energy calculations of the nanoribbons and Ag(111), respectively. K-point grids of 6×1×1 and 6×6×1 were used for DOS calculations of the nanoribbons and Ag(111), respectively. r-point was used for gas-phase molecules. For work function calculations of the monolayer transition metal dichalcogenide slabs, a 10×1×1 K-point grid was used. All calculations were spin-polarized calculations.
The effect of the CO coverage on the CO binding energies on the metal edges of the transition metal dichalcogenides was investigated. The DFT results show that each metal atom on the transition metal dichalcogenide nanoflake edge can bind up to two CO molecules (θco=2 ML). As shown in Table 8, the binding energies of CO on the metal edge decrease as the coverage increases. At the highest coverage (θco=2 ML), the average binding energy per second CO on the metal atom becomes smaller than 0.5 eV. This suggests that during the catalytic reaction, CO is likely to have a high coverage (θco>1 ML) on the metal edges of the transition metal dichalcogenides, and second CO molecule on the metal atom can easily desorb. These results indicate that the catalyst site may have at least one CO molecule binding the metal atom during most of the catalytic cycle.
This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/436,870, filed Dec. 20, 2017, which is hereby incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62436870 | Dec 2016 | US |
