Patent Application
20250101616
The present disclosure is directed to an electrode, particularly, a cerium-doped zeolitic imidazolate framework (Cey-ZIF-zni)-based electrode for electrocatalytic reduction of CO2 to CO.
The “background” description provided herein is to present the context of the disclosure generally. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
The continuing increase in the demand for sustainable energy is attracting significant global attention. However, the burning of fossil fuels, the primary energy source for industries and humans, leads to excessive greenhouse gas emissions (e.g., CO2, NO, and CH4) and thus accelerates climate change. To address these challenges, the potential of CO2 electrochemical reduction (CO2RR) has been investigated as a promising approach for environmentally friendly chemical production and energy storage infrastructure. Given CO2 electrochemical reduction's enormous economic and environmental potential, it has garnered the attention of researchers over the past decades. However, practical implementation of this technology poses various challenges. Ensuring product selectivity, enhancing conversion rates, and minimizing over-potential requirements are essential aspects that make the actualization of this technology a highly challenging task. This process involves numerous proton and electron transfers, resulting in the production of various products, each with several reaction intermediates. This complexity presents significant obstacles in characterizing molecular-level reaction mechanisms, which are crucial for designing selective, effective electrodes, and stable electro-catalysts.
In general, materials employed in electrochemical reduction should be available in large quantities with high efficiency while remaining cost-effective to ensure their economic viability. Different electrode materials and designs have been investigated for the electrochemical reduction of CO2. Some of these materials include metal nanostructures such as Cu, Au, Pt, and Ag, metal alloys such as Cu—Ni, Cu—Pd, Cu—Sn, Cu—Zn, Cu—Ag, and Cu—In, metal oxides and metal sulfides like CuOx, SnO2 and CeO2, chalcogenides, MoS2, WS2, and metal-free electrodes such as C and Si. Overall, the design (shape and size) of these electrodes affects their electrochemical behavior concerning CO2 reduction.
To address these challenges, transition metal and N—C (co-doped) (M/N—C) class of materials has been studied as electro-catalyst candidates for a range of electrochemical reactions, including carbon dioxide reduction reaction (CO2RR) [X.-L. Chen, L.-S. Ma, W.-Y. Su, L.-F. Ding, H.-B. Zhu, and H. Yang, “ZIF-derived bifunctional Cu@Cu—N—C composite electrocatalysts towards efficient electroreduction of oxygen and carbon dioxide,” Electrochim. Acta, vol. 331, p. 135273, 2020]. Typically, these materials are synthesized from suitable C, M, and N precursors under different temperatures and/or atmospheres (e.g., N2 and Ar). Various sources such as inorganic and/or organic compounds, metal-organic frameworks (MOFs), biomass, and organic polymer precursors have been utilized to control the structure, morphology, and microstructure of the resulting M/N—C materials. Amongst these materials, zeolitic imidazole frameworks (ZIFs), a subclass of MOFs composed of M-Im-M (where M denotes Zn, Co cation, and Im represents the imidazolate linker), have emerged as an attractive group of self-sacrificed support for highly effective M/N—C electro-catalysts. This is due to properties such as high surface area, tunable pores and structures, and expanded configurations [Z. Lian, L. Huimin, and Y. Lisha, “Microwave ionothermal synthesis of ZIF-61 and its application on the curing process of cyanate ester (CE),” Mater. Lett., vol. 125, pp. 59-62, 2014, B. Chen, Z. Yang, Y. Zhu, and Y. Xia, “Zeolitic imidazolate framework materials: recent progress in synthesis and applications,” J. Mater. Chem. A, vol. 2, no. 40, pp. 16811-16831, 2014]. These characteristics have sparked interest for potential industrial applications, including adsorption and separation, drug delivery, sensing, and as catalyst or catalyst supports.
However, the widespread industrial application of ZIFs is hindered by certain challenges, including cost and energy, as well as time consuming synthetic procedures, resulting in a space-time yield typically below 300 kg m−3 d−1. This elevated cost poses a barrier to large-scale industrial production and commercial implementation. Moreover, traditional methods still suffer from high energy consumption and long reaction times (>24 h) with low production rates. To tackle these challenges, approaches have been described in the present disclosure to improve the synthesis efficiency of ZIFs and enhance their industrial feasibility, and to overcome the drawbacks of the art.
Ceria, an abundant Earth's ore, finds numerous applications as a redox material in catalytic processes. Owing to ceria's ability to store and discharge oxygen, leading to the formation of Ce3+, and maintaining single-phase stability at elevated temperature, ceria is attractive for various catalytic functions. Additionally, it can further aid water separation by forming adsorbed hydrogen (H*) due to its negative binding affinity with H*. On the other hand, the formation of frustrated-Lewis pairs in CeO2 may promote the simultaneous adsorption and activation of CO2. The oxygen lattice binds with carbon, while the two nearby Ce3+ ions may bond to oxygen atoms of CO2 as the Lewis basic site and Lewis acidic site, respectively. However, its direct application as electro-catalysts is limited due to its inferior electron-ion conductivity.
In view of the foregoing, it is one objective of the present disclosure to provide an electrocatalyst for CO2 reduction. A second objective of the present disclosure is to provide an electrode containing the electrocatalyst. A third objective of the present disclosure is to provide a method of making the electrode containing the electrocatalyst. A fourth objective of the present disclosure is to provide a method of electrochemical carbon dioxide (CO2) reduction (CO2RR) using the electrode.
In an exemplary embodiment, an electrode is described. The electrode includes a transparent glass substrate, a cerium-doped zeolitic imidazolate framework (Cey-ZIF-zni) material at least partially covering a surface of the transparent glass substrate. In some embodiments, the Cey-ZIF-zni material is at least one of an excess solvent deposition (ESD) Cey-ZIF-zni material (Cey-ZIF-zni-X), and a chemical vapor deposition (CVD) Cey-ZIF-zni material (Cey-ZIF-zni-CVD). In some embodiments, y is between 0.01 and 1 inclusive. In some embodiments, the Cey-ZIF-zni-X includes flake-like particles have an average particle size of 2 to 7 micrometers (μm). In some embodiments, the Cey-ZIF-zni-CVD contains rod-like particles having an average particle size of 1 to 6 μm. In some embodiments, the Cey-ZIF-zni material is uniformly disposed on the surface of the transparent glass substrate.
In some embodiments, the transparent substrate is a glass substrate selected from the group consisting of a fluorine doped tin oxide (FTO) glass substrate, a tin doped indium oxide (ITO) glass substrate, an aluminum doped zinc oxide (AZO) glass substrate, a niobium doped titanium dioxide (NTO) glass substrate, an indium doped cadmium oxide (ICO) glass substrate, an indium doped zinc oxide (IZO) glass substrate, a fluorine doped zinc oxide (FZO) glass substrate, a gallium doped zinc oxide (GZO) glass substrate, an antimony doped tin oxide (ATO) glass substrate, a phosphorus doped tin oxide (PTO) glass substrate, a zinc antimonate glass substrate, a zinc oxide glass substrate, a ruthenium oxide glass substrate, a rhenium oxide glass substrate, a silver oxide glass substrate, and a nickel oxide glass substrate.
In some embodiments, the Cey-ZIF-zni-CVD has an average particle size of about 3.2 μm, and a lattice interatomic spacing of the (111) plane of about 0.28 nm.
In an exemplary embodiment, a method of making the electrode containing the Cey-ZIF-zni material is described. In some embodiments, the Cey-ZIF-zni material is Cey-ZIF-zni-X. The method further includes preparing the Cey-ZIF-zni-X by mixing a zinc salt, an imidazole, and an alcohol to form a first mixture; heating the first mixture thereby reacting the zinc salt with the imidazole to form a first ZIF-zni precursor in the first mixture; removing the ZIF-zni precursor from the first mixture, washing, and heating to form the ZIF-zni material; mixing the ZIF-zni material, a cerium salt and the alcohol and drying to form a second mixture; and heating the second mixture at a temperature of about 150° C. thereby generating cerium nanoparticles disposed on surfaces of the ZIF-zni material to form the Cey-ZIF-zni-X. In some embodiments, a weight ratio of the ZIF-zni material to the cerium salt is in a range of 20:1 to 1:1.
In some embodiments, the zinc salt is at least one selected from the group consisting of zinc sulfate, zinc acetate, zinc citrate, zinc iodide, zinc chloride, zinc perchlorate, zinc nitrate, zinc phosphate, zinc triflate, zinc bis(trifluoromethylsulfonyl)imide, zinc tetrafluoroborate, and zinc bromide, or its hydrate, or a mixture of two or more of any of these.
In some embodiments, the imidazole has a formula (I)
in which R1, R2, and R3 are each independently selected from the group consisting of a hydrogen, an optionally substituted alkyl, an optionally substituted cycloalkyl, an optionally substituted alkoxy, a hydroxyl, a halogen, a nitro, and a cyano.
In some embodiments, the alcohol is at least one of methanol, ethanol, iso-propanol.
In some embodiments, the ZIF-zni material has an average particle size of about 4.3 μm.
In some embodiments, the cerium salt is at least one of cerium chloride, cerium nitrate, cerium ammonium nitrate, and cerium sulfate, or its hydrate, or a mixture of two or more of any of these.
In an exemplary embodiment, a method of making the electrode containing the Cey-ZIF-zni material is described. In some embodiments, the Cey-ZIF-zni material is Cey-ZIF-zni-CVD. The method further includes preparing the Cey-ZIF-zni-CVD by mixing a zinc salt, an imidazole, and an alcohol to form a first mixture; heating the first mixture thereby reacting the zinc salt with the imidazole to form a first ZIF-zni precursor in the first mixture; removing the ZIF-zni precursor from the first mixture, washing and heating to form the ZIF-zni material; and generating a cerium precursor vapor by heating a cerium salt and passing the cerium precursor vapor in contact with the ZIF-zni material thereby generating cerium nanoparticles disposed in the lattices and surfaces of the ZIF-zni material to form the Cey-ZIF-zni-CVD. In some embodiments, a weight ratio of the ZIF-zni material to the cerium salt is in a range of 20:1 to 1:1.
In some embodiments, the zinc salt is at least one selected from the group consisting of zinc sulfate, zinc acetate, zinc citrate, zinc iodide, zinc chloride, zinc perchlorate, zinc nitrate, zinc phosphate, zinc triflate, zinc bis(trifluoromethylsulfonyl)imide, zinc tetrafluoroborate, and zinc bromide, or its hydrate, or a mixture of two or more of any of these.
In some embodiments, the imidazole has a formula (I)
in which R1, R2, and R3 are each independently selected from the group consisting of a hydrogen, an optionally substituted alkyl, an optionally substituted cycloalkyl, an optionally substituted alkoxy, a hydroxyl, a halogen, a nitro, and a cyano.
In some embodiments, the alcohol is at least one of methanol, ethanol, iso-propanol.
In some embodiments, the ZIF-zni material has an average particle size of about 4.3 μm.
In some embodiments, the cerium salt is at least one of cerium chloride, cerium nitrate, cerium ammonium nitrate, and cerium sulfate, or its hydrate, or a mixture of two or more of any of these.
In some embodiments, the cerium salt is heated in a furnace at a heating rate of 1 to 4 degree Celsius per minute (° C. min−1) in an inert atmosphere until the temperature of the furnace reaches a temperature of about 200° C.
In an exemplary embodiment, a method of electrochemical CO2 reduction (CO2RR) is described. The method includes charging an electrolyte to an electrochemical cell containing a working electrode, a counter electrode, and a reference electrode; introducing a CO2-containing gas composition into the electrochemical cell containing the electrolyte and passing the CO2-containing gas composition through the electrolyte; during the passing, simultaneously applying a potential between the working electrode and the counter electrode in the electrochemical cell via the electrolyte to form carbon monoxide (CO). In some embodiments, the working electrode is the electrode of the present disclosure. In some embodiments, the electrolyte includes an aqueous solution of a salt having a concentration of 0.05 to 2 M.
In some embodiments, the salt includes potassium bicarbonate (KHCO3), and sodium bicarbonate (NaHCO3).
In some embodiments, CO2 is present in the CO2-containing gas composition at a concentration of at least 90 wt. % based on a total weight of the CO2-containing gas composition.
In some embodiments, the method of electrochemical CO2 reduction has a CO faradaic efficiency (FE) of about 88% based on the total charge passed at 0.7 volts (V) vs RHE.
In some embodiments, the method of electrochemical CO2 reduction has an over potential of 170 millivolts (mV) and a CO onset of −300 mV.
The foregoing general description of the illustrative present disclosure and the following The detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.
A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In the drawings, reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words “a,” “an,” and the like generally carry a meaning of “one or more,” unless stated otherwise.
Furthermore, the terms “approximately,” “approximate,” “about,” and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.
As used herein, the term “particle size” generally refers to the distance between two opposite points on a particle. In the present disclosure, the “particle size” may refer to the length or longest dimension of a particle.
As used herein, the term “electrode” generally refers to a conductor through which electricity enters or leaves a device or material. In the present disclosure, the “electrode” may be an electrical conductor used to contact a non-metallic part of a circuit e.g., a semiconductor, an electrolyte, a vacuum, or air.
As used herein, the term “current density” generally refers to the amount of electric current traveling per unit cross-section area.
As used herein, the term “Tafel slope” generally refers to the relationship between the overpotential and the logarithmic current density.
As used herein, the term “electrochemical cell” generally refers to a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions.
As used herein, the term “inter-atomic spacing” generally refers to the distance between the parallel planes of atoms. In the present disclosure, the term “inter-atomic spacing” may be the minimum distance between two planes.
As used herein, the term “overpotential” generally refers to the difference in potential which exists between a thermodynamically determined reduction potential of a half-reaction and the potential at which the redox event is experimentally observed. The term is directly associated to a cell's voltage efficacy. In an electrolytic cell, the occurrence of overpotential implies that the cell needs more energy as compared to that thermodynamically expected to drive a reaction. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is measured by determining the potential at which a given current density is reached.
As used herein, the term “substituted” generally refers to at least one hydrogen atom that is replaced with a non-hydrogen group, provided that normal valences are maintained and that the substitution results in a stable compound. When a substituent is noted as “optionally substituted”, the substituents are selected from the exemplary group including, but not limited to, halo, hydroxyl, alkoxy, oxo, alkanoyl, aryloxy, alkanoyloxy, amino, alkylamino, arylamino, arylalkylamino, disubstituted amines (e.g. in which the two amino substituents are selected from the exemplary group including, but not limited to, alkyl, aryl or arylalkyl), alkanylamino, aroylamino, aralkanoylamino, substituted alkanoylamino, substituted arylamino, aubstituted aralkanoylamino, thiol, alkylthio, arylthio, arylalkylthio, alkylthiono, arylthiono, aryalkylthiono, alkylsulfonyl, arylsulfonyl, arylalkylsulfonyl, sulfonamide (e.g. —SO2NH2), substituted sulfonamide, nitro, cyano, carboxy, carbamyl (e.g. —CONH2), substituted carbamyl (e.g. —CONHalkyl, —CONHaryl, —CONHarylalkyl or cases where there are two substituents on one nitrogen from alkyl, aryl, or alkylalkyl), alkoxycarbonyl, aryl, substituted aryl, guanidine, heterocyclyl (e.g. indolyl, imidazoyl, furyl, thienyl, thiazolyl, pyrrolidyl, pyridyl, pyrimidiyl, pyrrolidinyl, piperidinyl, morpholinyl, piperazinyl, homopiperazinyl and the like), substituted heterocyclyl and mixtures thereof and the like. The substituents may themselves be optionally substituted and may be either unprotected or protected as necessary, as known to those skilled in the art, for example, as taught.
As used herein, the term “alkyl” unless otherwise specified refers to both branched and straight chain saturated aliphatic primary, secondary, and/or tertiary hydrocarbons of typically C1 to C20, preferably C6-C18, more preferably C10-C16, for example C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, and specifically includes, but is not limited to, methyl, trifluoromethyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, t-butyl, pentyl, cyclopentyl, isopentyl, neopentyl, hexyl, isohexyl, cyclohexyl, cyclohexylmethyl, 3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 2-ethylhexyl, heptyl, octyl, nonyl, 3,7-dimethyloctyl, decyl, undecyl, dodecyl, tridecyl, 2-propylheptyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, and eicosyl. As used herein, the term optionally includes substituted alkyl groups. Exemplary moieties with which the alkyl group can be substituted may be selected from the group including, but not limited to, hydroxyl, amino, alkylamino, arylamino, alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid, phosphate, halo, or phosphonate or mixtures thereof. The substituted moiety may be either protected or unprotected as necessary, and as known to those skilled in the art.
The term “cycloalkyl” refers to cyclized alkyl groups. Exemplary cycloalkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, norbornyl, and adamantyl. Branched cycloalkyl groups, such as exemplary 1-methylcyclopropyl and 2-methylcyclopropyl groups, are included in the definition of cycloalkyl as used in the present disclosure.
The term “alkoxy” refers to a straight or branched chain alkoxy including, but not limited to, methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, secondary butoxy, tertiary butoxy, pentoxy, isopentoxy, hexyloxy, heptyloxy, octyloxy, nonyloxy, and decyloxy.
The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.
Aspects of the present disclosure are directed to a method of making zeolitic imidazolate framework (ZIF-zni) interface, for example, preferably at reflux temperature in 3 hours. Subsequently, cerium was incorporated into the as-synthesized ZIF-zni via excess solvent (ESD) and chemical vapor deposition (CVD) approach and applied as an electrocatalyst for CO2 reduction. The electrocatalyst of the present disclosure, which can be used alone or in combination with another material, e.g., a transparent glass substrate, can effectively transform CO2 to CO products at room temperature.
In one exemplary embodiment, an electrode is described. The electrode includes a transparent glass substrate. In some embodiments, the transparent substrate is a glass substrate. In some further embodiments, the glass substrate is one of a fluorine doped tin oxide (FTO) glass substrate, a tin doped indium oxide (ITO) glass substrate, an aluminum doped zinc oxide (AZO) glass substrate, a niobium doped titanium dioxide (NTO) glass substrate, an indium doped cadmium oxide (ICO) glass substrate, an indium doped zinc oxide (IZO) glass substrate, a fluorine doped zinc oxide (FZO) glass substrate, a gallium doped zinc oxide (GZO) glass substrate, an antimony doped tin oxide (ATO) glass substrate, a phosphorus doped tin oxide (PTO) glass substrate, a zinc antimonate glass substrate, a zinc oxide glass substrate, a ruthenium oxide glass substrate, a rhenium oxide glass substrate, a silver oxide glass substrate, and a nickel oxide glass substrate. In some preferred embodiments, the glass substrate is one of an FTO glass substrate, and an ITO glass substrate.
Additionally, the electrode further includes a cerium-doped zeolitic imidazolate framework (Cey-ZIF-zni) material that at least partially covers a surface of the transparent glass substrate. In some embodiments, the transparent glass substrate is in a sheet-form having two sides. In some embodiments, only one of the two sides of the transparent glass substrate is covered with the Cey-ZIF-zni material. In some embodiments, the Cey-ZIF-zni material covers at least 50%, preferably 60%, preferably 70%, preferably 80%, preferably 90%, preferably 95%, preferably greater than 99% of the transparent glass substrate, each % based on a total surface area of one side of the transparent glass substrate. In some other embodiments, both sides of the transparent glass substrate are covered with the Cey-ZIF-zni material. In some further embodiments, the Cey-ZIF-zni material covers at least 50%, preferably 60%, preferably 70%, preferably 80%, preferably 90%, preferably 95%, preferably greater than 99% of the transparent glass substrate, each % based on a total surface area of the transparent glass substrate. In some embodiments, the Cey-ZIF-zni material is preferably uniformly disposed on a surface of the transparent glass substrate. In some embodiments, the Cey-ZIF-zni material (Cey-ZIF-zni-X), may be prepared by an excess solvent deposition (ESD) method. In some embodiments, the Cey-ZIF-zni material (Cey-ZIF-zni-CVD), may be prepared by chemical vapor deposition (CVD) method. In some embodiments, the ‘y’ in the Cey-ZIF-zni material is between 0.01 and 1 inclusive, preferably between 0.03 to 0.8 inclusive, preferably between 0.06 to 0.6 inclusive, or even more preferably between 0.23 to 0.4. Other ranges are also possible.
Referring to
As used herein, the term “inter-particle voids” generally refers to the spaces or gaps between particles in a material or substance. In the present disclosure, the “inter-particle voids” may be the spaces between the nanoparticles in the ZIF-zni material, the Cey-ZIF-zni-X, and the Cey-ZIF-zni-CVD, respectively. In some embodiments, a total volume of the inter-particle voids of the Cey-ZIF-zni-X is nor more than 80%, preferably no more than 60%, preferably no more than 40%, or even preferably no more than 20% by volume of the total volume of the inter-particle voids of the ZIF-zni material. In some further embodiments, a total volume of the inter-particle voids of the Cey-ZIF-zni-CVD is no more than 90%, preferably no more than 70%, preferably no more than 50%, or even preferably no more than 30% by volume of the total volume of the inter-particle voids of the ZIF-zni material. Other ranges are also possible.
The Cey-ZIF-zni particles include particles in the form of flakes and rods. In some embodiments, the particles may exist in other morphological forms such as nanowires, nanocrystals, nanorectangles, nanotriangles, nanopentagons, nanohexagons, nanoprisms, nanodisks, nanocubes, nanoribbons, nanoblocks, nanobeads, nanotoroids, nanodiscs, nanobarrels, nanogranules, nanowhiskers, nanoflakes, nanofoils, nanopowders, nanoboxes, nanostars, tetrapods, nanobelts, nano-urchins, nanofloweres, and mixtures thereof.
Referring to
Referring to
At step 52, the method 50 includes mixing a zinc salt, an imidazole, and an alcohol to form a first mixture. In some embodiments, the zinc salt is one or more of zinc sulfate (ZnSO4), zinc acetate (Zn(CH3CO2)2), zinc citrate (C12H10O14Zn3), zinc iodide (ZnI2), zinc chloride (ZnCl2), zinc perchlorate (Zn(ClO4)2), zinc nitrate (Zn(NO3)2·6H2O), zinc phosphate (Zn3(PO4)2), zinc triflate (C2F6O6S2Zn), zinc bis(rifluoromethylsulfonyl)imide (C4F12N2O8S4Zn), zinc tetrafluoroborate (Zn(BF4)2), and zinc bromide (ZnBr2), or its hydrate, or a mixture of two or more of any of these. In a preferred embodiment, the zinc salt is zinc nitrate, or its hydrate (Zn(NO3)2·6H2O).
In some embodiments, the imidazole has a formula (I)
in which R1, R2, and R3 are each independently selected from the group consisting of a hydrogen, an optionally substituted alkyl, an optionally substituted cycloalkyl, an optionally substituted alkoxy, a hydroxyl, a halogen, a nitro, and a cyano. In a preferred embodiment, the molar ratio of Zn (NO3)2 to imidazole is in a range of 1:20 to 20:1, preferably 1:15 to 15:1, preferably 14:1 to 1:14, preferably 13:1 to 1:13, preferably 1:12 to 12:1, preferably 11:1 to 1:11, preferably 1:10. Other ranges are also possible. In some embodiments, the alcohol is one or more of methanol, ethanol, iso-propanol. Optionally, other lower alcohols (C1-C6) may be used as well. In a preferred embodiment, the alcohol is methanol.
At step 54, the method 50 includes heating the first mixture, thereby reacting the zinc salt with the imidazole to form a first ZIF-zni precursor in the first mixture. In an embodiment, the mixture is heated for about 1-4 hours, preferably 2-3 hours, preferably 2 hours, at a temperature range of 50-100° C., preferably 60-90° C., preferably 65-70° C. In a specific embodiment, the mixture was refluxed for about 2 h at 65° C. while stirring. Other ranges are also possible.
At step 56, the method 50 includes removing the ZIF-zni precursor from the first mixture, washing, and heating to form the ZIF-zni material. The ZIF-zni precursor may be removed from the first mixture via filtration. The ZIF-zni precursor may be further washed several times with methanol to remove impurities followed by heating at a temperature range of 50-100° C., preferably 55-95° C., 60-90° C., preferably 65-85° C., preferably 65° C. for 8-12 hours. Other ranges are also possible. In a preferred embodiment, the heating may be carried out under a vacuum to prevent oxidation. In some embodiments, the ZIF-zni material has an average particle size of about 3 to 5.5 μm, or even more preferably about 4.3 μm. Other ranges are also possible.
At step 58, the method 50 includes mixing the ZIF-zni material, a cerium salt and the alcohol and drying to form a second mixture. The mixing may be carried out manually or with the help of a stirrer. In some embodiments, the cerium salt is at least one of cerium chloride (CeCl3), cerium nitrate (Ce(NO3)3), cerium ammonium nitrate ((NH4)2[Ce(NO3)6]), and cerium sulfate (Ce(SO4)2), or its hydrate, or a mixture of two or more of any of these. In a preferred embodiment, the cerium salt is cerium nitrate, or its hydrate (Ce (NO3)3·6H2O).
At step 60, the method 50 includes heating the second mixture at a temperature of about 150° C. thereby generating cerium nanoparticles disposed on surfaces of the ZIF-zni material to form the Cey-ZIF-zni-X. In some embodiments, a weight ratio of the ZIF-zni material to the cerium salt is in a range of 20:1-1:1, preferably 20:1, preferably 19:1, preferably 18:1, preferably 17:1, preferably 16:1, preferably 15:1, preferably 14:1, preferably 13:1, preferably 12:1, preferably 11:1, preferably 10:1, preferably 9:1, preferably 8:1, preferably 7:1, preferably 6:1, preferably 5:1, preferably 4:1, preferably 3:1, preferably 2:1, and preferably 1:1. In a preferred embodiment, the weight ratio of the ZIF-zni material to the cerium salt is 1:1. Other ranges are also possible.
At step 72, the method 70 includes mixing a zinc salt, an imidazole, and an alcohol to form a first mixture. The zinc salt is at least one selected from the group consisting of ZnSO4, Zn(CH3CO2)2, C12H10O14Zn3, ZnI2, ZnCl2, Zn(ClO4)2, Zn(NO3)2, Zn3(PO4)2, C2F6O6S2Zn, C4F12N2O8S4Zn, Zn(BF4)2, and ZnBr2, or its hydrate, or a mixture of two or more of any of these. In a preferred embodiment, the zinc salt is zinc nitrate, or is hydrate(Zn (NO3)2·6H2O).
In some embodiments, the imidazole has a formula (I)
in which, R1, R2, and R3 are each independently selected from the group consisting of a hydrogen, an optionally substituted alkyl, an optionally substituted cycloalkyl, an optionally substituted alkoxy, a hydroxyl, a halogen, a nitro, and a cyano. In a preferred embodiment, the molar ratio of Zn (NO3)2 to imidazole is in a range of 1:20 to 20:1, preferably 1:15 to 15:1, preferably 14:1 to 1:14, preferably 13:1 to 1:13, preferably 1:12 to 12:1, preferably 11:1 to 1:11, preferably 1:10. Other ranges are also possible. The alcohol is one or more of methanol, ethanol, iso-propanol. Optionally, other lower alcohols (C1-C6) may be used as well. In a preferred embodiment, the alcohol is methanol.
At step 74, the method 70 includes heating the first mixture thereby reacting the zinc salt with the imidazole to form a first ZIF-zni precursor in the first mixture. In an embodiment, the mixture is heated for about 1-4 hours, preferably 2-3 hours, preferably 2 hours, at a temperature range of 50-100° C., preferably 60-90° C., preferably 65-70° C. In a specific embodiment, the mixture was refluxed for about 2 h at 65° C. while stirring. Other ranges are also possible.
At step 76, the method 70 includes removing the ZIF-zni precursor from the first mixture, washing, and heating to form the ZIF-zni material. The ZIF-zni precursor may be removed from the first mixture via filtration. The ZIF-zni precursor may be further washed several times with methanol to remove impurities followed by heating at a temperature range of 50-100° C., preferably 55-95° C., 60-90° C., preferably 65-85° C., preferably 65° C. for 8-12 hours. Other ranges are also possible. In a preferred embodiment, the heating may be carried out under a vacuum to prevent oxidation.
At step 78, the method 70 includes generating a cerium precursor vapor by heating a cerium salt, and passing the cerium precursor vapor in contact with the ZIF-zni material thereby generating cerium nanoparticles disposed in the lattices and surfaces of the ZIF-zni material to form the Cey-ZIF-zni-CVD. In some embodiments, the cerium salt is at least one of CeCl3, Ce(NO3)3, (NH4)2[Ce(NO3)6], and Ce(SO4)2, or its hydrate, or a mixture of two or more of any of these. In a preferred embodiment, the cerium salt is Ce(NO3)3, or its hydrate (Ce (NO3)3·6H2O). The chemical vapor deposition (CVD) method was explored for the effective synthesis of the catalyst. For this purpose, the cerium salt is heated in a furnace at a heating rate of 1-4 degree Celsius per minute (° C. min−1), preferably 1, preferably 2, preferably 3, preferably 4° C. min−1, in an inert atmosphere until the temperature of the furnace reaches a temperature of about 200° C. In some embodiments, the cerium salt is heated in a furnace at a heating rate of 1-4 degree Celsius per 10 minute (° C. min−1), preferably 1, preferably 2, preferably 3, preferably 4° C. min−1, in an inert atmosphere until the temperature of the furnace reaches a temperature of about 200° C. In a preferred embodiment, the cerium salt is heated in a furnace at a heating rate of 2° C. min−1. Other ranges are also possible. The choice for 200° C. as the heating temperature, was considered due to the melting point of the cerium precursor. In some embodiments, the inert atmosphere can be created by Argon (Ar), nitrogen (N), Helium (He) flow. In a preferred embodiment, the inert atmosphere was created by Ar flow.
At step 80, the method 70 includes heating the second mixture at a temperature of about 150° C. thereby generating cerium nanoparticles disposed on surfaces of the ZIF-zni material to form the Cey-ZIF-zni-CVD. In some embodiments, a weight ratio of the ZIF-zni material to the cerium salt is in a range of 20:1-1:1, preferably 20:1, preferably 19:1, preferably 18:1, preferably 17:1, preferably 16:1, preferably 15:1, preferably 14:1, preferably 13:1, preferably 12:1, preferably 11:1, preferably 10:1, preferably 9:1, preferably 8:1, preferably 7:1, preferably 6:1, preferably 5:1, preferably 4:1, preferably 3:1, preferably 2:1, and preferably 1:1. Other ranges are also possible. In a preferred embodiment, the weight ratio of the ZIF-zni material to the cerium salt is 1:1. Other ranges are also possible.
The crystalline structures of the ZIF-zni material, Cey-ZIF-zni-X, and Cey-ZIF-zni-CVD may be characterized by X-ray diffraction (XRD). In some embodiments, the XRD patterns are collected in a Rigaku Miniflex 600 X-ray diffractometer equipped with a Cu-Kα radiation source (X=1.5406 Å) for a 20 range extending between 1° and 80°, preferably 20 and 70°, further preferably 30 and 600 at an angular rate of 0.005 to 0.04° s−1, preferably 0.01 to 0.03° s−1, or even preferably 0.02° s−1.
In some embodiments, the ZIF-zni material has peaks with a 20 value of about 12 to 15°, about 15 to 17°, about 17 to 22°, about 23 to 27°, about 27 to 30°, about 30 to 34°, about 35 to 36°, and about 41 to 43°, in the XRD spectrum, as depicted in
The structures of the ZIF-zni material, Cey-ZIF-zni-X, and Cey-ZIF-zni-CVD may be characterized by Fourier-transform infrared spectroscopy (FT-IR), respectively. In some embodiments, the FT-IR are collected in a Nicolet 6700 series acquired in a range of 4500 to 400 centimeter inverse (cm−1) at 4 cm−1 resolution. At least 5, at least 10, or preferably at least 20 scans were carried out for each sample. In some embodiments, the ZIF-zni material has peaks at 500 to 850 cm−1, 900 to 1200 cm−1 and 1300 to 1700 cm−1 in the FT-IR spectrum, confirming its formation as depicted in
At step 92, the method 90 includes charging an electrolyte to an electrochemical cell comprising a working electrode, a counter electrode, and a reference electrode. A reference electrode is an electrode that has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each relevant species of the redox reaction. A reference electrode may enable a potentiostat to deliver a stable voltage to the working electrode or the counter electrode. The reference electrode may be a standard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), a reversible hydrogen electrode (RHE), a saturated calomel electrode (SCE), a copper-copper(II) sulfate electrode (CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode, a palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), a mercury-mercurous sulfate electrode, or some other type of electrode. In a preferred embodiment, a reference electrode is present in the electrochemical cell and is a silver chloride electrode (Ag/AgCl).
In some embodiments, where the counter electrode includes platinum, the counter electrode is in the form of a rod or wire. Alternatively, the counter electrode may comprise some other electrically-conductive material such as platinum-iridium alloy, iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloy, and/or some other electrically-conductive material, where an “electrically-conductive material” as defined here is a substance with an electrical resistivity of at most 10−6 Ω·m, preferably at most 10−7 Ω·m, more preferably at most 10−8 Ω·m at a temperature of 20-25° C. The working electrode is the electrode including a transparent glass substrate and Cey-ZIF-zni. In some embodiments, the electrolyte includes an aqueous solution of a salt having a concentration of 0.05-2 M, preferably 0.05, preferably 0.1, preferably 0.5, preferably 1.0, preferably 1.5, preferably 2 M. In a preferred embodiment, the electrolyte includes the aqueous solution of a salt having a concentration of 0.5 M. Other ranges are also possible. Suitable examples of electrolyte salts are sodium chloride (NaCl), potassium chloride (KCl), potassium bicarbonate (KHCO3), and sodium bicarbonate (NaHCO3). In some embodiments, the salt involves KHCO3, and NaHCO3. In a preferred embodiment, the salt includes KHCO3.
The water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In a preferred embodiment, water is deionized water with the conductivity 10 S cm−1.
Preferably, to maintain uniform concentrations and/or temperatures of the electrolyte solution, the electrolyte solution may be stirred or agitated during the step of the subjecting. The stirring or agitating may be done intermittently or continuously. This stirring or agitating may be done by a magnetic stir bar, a stirring rod, an impeller, a shaking platform, a pump, a sonicator, a gas bubbler, or some other device. Preferably the stirring is achieved by an impeller or a magnetic stir bar.
At step 94, the method 90 includes introducing a CO2-containing gas composition into the electrochemical cell containing the electrolyte and passing the CO2-containing gas composition through the electrolyte. In some embodiments, CO2 is present in the CO2-containing gas composition at a concentration of at least 50 wt. %, preferably at least 60 wt. %, preferably at least 70 wt. %, preferably at least 80 wt. %, or even more preferably at least 90 wt. % based on the total weight of the CO2-containing gas composition. Other ranges are also possible.
At step 96, the method 90 includes simultaneously applying a potential between the working electrode and the counter electrode in the electrochemical cell via the electrolyte to form carbon monoxide (CO) during the passing. The CO faradaic efficiency (FE) observed for Ce0.23-ZIF-zni-CVD electrode was about 88% based on the total charge passed at 0.7 volts (V) vs RHE, as depicted in
Referring to
The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.
The following examples demonstrate a cerium-doped zeolitic imidazolate framework (Cey-ZIF-zni) material-based electrode. The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.
Ce(NO3)3·6H2O (AR, ≥99.0%), Zn(NO3)2·6H2O (AR, ≥98.5%), and imidazole were purchased from Sigma-Aldrich while methanol (AR, ≥99.0%) was obtained from obtained from Acros Organics and all chemicals were used as received. High-purity deionized water (18 S/cm) was produced and obtained in-house using a Thermo Scientific Barnstead Nanopure water purification system after distillation with a Labstrong FiSTREEM II 2S Glass Still distiller.
ZIF-zni was prepared as follows, about 0.006 mol of Zn (NO3)2·6H2O in 50 mL methanol was added into 50 mL of methanol containing 0.06 mol of imidazole under vigorous stirring for 1 h at room temperature and the mixture was refluxed for about 2 h at 65° C. while stirring. The solid particles were collected by centrifuge and washed several times with methanol. The final whitish powder was oven-dried under vacuum at 65° C. overnight. The ZIF-zni powder was activated at 200° C. under vacuum for 2 h prior to use.
ZIF-zni supported CeyOy nanoparticles (Cey-ZIF-zni-X) were first of all synthesized by dispersing ZIF-zni and Ce (NO3)3·6H2O in methanol, as described herein. Typically, 1:1 ZIF-zni and Ce precursor were dispersed in 20 mL of methanol and sonicated for 0.5 h at room temperature, followed by oven drying at 60° C. overnight. The final powdered products were activated at 150° C. and denoted as Ce-ZIF-zni. Furthermore, chemical vapor deposition (CVD) method was explored for the effective synthesis of the catalyst. In a typical preparation, 1:1 ratio of ZIF-zni and Ce precursor were placed separately in two porcelain boats and heated in a tubular furnace to 200° C. for 2 h with a heating rate of 2° C. min−1 under Ar flow and the final products were denoted as Ce-ZIF-zni-CVD. The choice for 200° C. as the heating temperature, was considered due to the melting point of the cerium precursor. To determine the optimal ratio of Ce in Ce-ZIF-zni-X (excess solvent (ESD)), and in Ce-ZIF-zni-CVD (chemical vapor deposition (CVD)), respectively, for the electrochemical CO2 reduction to CO, three different percentage of Ce(NO3)3·6H2O were synthesis and denoted as Cey-ZIF-zni (where y is the mol of Ce).
The X-ray diffraction patterns of the ZIF-zni, correlates well with the reported experimental and simulated crystal data, as shown in
The formation of ZIF-zni was further confirmed by Fourier transform infrared spectroscopy (FTIR), as shown in
The crystal morphologies of the synthesized catalysts were examined by scanning electron microscopy (SEM). The SEM images of ZIF-zni, depicted in
The observed rod-like structure of the ZIF-zni in SEM could also be noticed with HRTEM analysis as depicted in
The electro-catalytic activity of CO2 reduction on Cey-ZIF-zni catalysts were examined by linear sweep voltammetry measurements (LSV) in 0.5 M KHCO3 solution saturated with CO2 and Ar with parent ZIF-zni as the reference electro-catalyst. As presented in
The electrochemical active surface areas (ECSA) of the ZIF-zni and Cey-ZIF-zni electro-catalysts were measured from the CV scans of 10-100 mVs1 rate in 0.5 M KHCO3 saturated CO2 solution
The electro-catalytic activity of both the parent ZIF-zni and Cey-ZIF-zni for CO2 electrochemical reduction reaction were investigated in 0.5 M KHCO3 saturated CO2 environment at −0.7 V to −1.0 V vs RHE. As presented in
xPdTexCex/C
The electrochemical kinetics of the reaction process was examined to have an impact on reaction mechanism. As presented in
An effective method for synthesizing ZIF-zni under ambient conditions with low solvent usage is disclosed. The resulting ZIF-zni maintains its rod-like morphology in the presence of the cerium. The Ce-doped ZIF-zni catalysts showed improved electrocatalytic performance on CO2 reduction to CO, achieving a faradaic efficiency of up to 87.9% on Ce0.23-ZIF-zni-CVD. The electrocatalytic performance is attributed to the number of catalytic sites on the catalysts. These results showed the potential of Ce0.23-ZIF-zni-CVD for transforming CO2 into value-added chemicals.
Numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
