Patent Application
20250137146
Aspects of the present disclosure were described in an article titled “Silver-Doped Zeolitic Imidazolate Framework (Ag@ZIF-8): An Efficient Electrocatalyst for CO2 Conversion to Syngas” published in Issue 13, Volume 867, Catalysts, which is incorporated herein by reference in its entirety.
Support provided by the Interdisciplinary Research Center for Hydrogen and Energy Storage (IRC-HES), King Fahd University of Petroleum and Minerals, Saudi Arabia is gratefully acknowledged.
Support provided by the Saudi Aramco Chair Professor Project, King Fahd University of Petroleum and Minerals, Saudi Arabia, through Project ORCP2390 is gratefully acknowledged.
The present disclosure is directed to an electrocatalyst for converting carbon dioxide to syngas, and particularly to a method for converting carbon dioxide to syngas using a silver-doped zeolitic imidazolate framework (Ag@ZIF-8)-based electrocatalyst.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Syngas, also referred to as the synthetic gas, includes carbon monoxide (CO) and diatomic hydrogen (H2). Steam-reforming methane, partial oxidation of hydrocarbons, and gasification of biomass, as well as various other methods, can be used to prepare syngas. Syngas is a fuel gas mixture that can be used in many ways. Syngas has the ability to make several products, including chemicals, fuels, and electricity. Compositions of syngas that consist of varying volumetric ratios of H2/CO in quantities of 33.33/66.67, 50/50, 66.67/33.33, 80/20, and 100/0 can be used as precursors to make synthetic fuels, such as natural gas, methanol, and dimethyl ether, via a Fischer-Tropsch (FT) process. In addition, syngas can be used as an alternative to fossil fuels because it can be made from different feedstocks, such as biomass and waste feedstock, and can be further used in gas turbines to produce electricity, which generates less greenhouse gas overall.
One such greenhouse gas is carbon dioxide (CO2), a colorless and odorless compound that is part of the carbon cycle. One of the causes of the world's climate change is the rising level of CO2 in the atmosphere. The current concentration of CO2 in the atmosphere is at about 415 parts per million (ppm). This high level of CO2 in the air is a detriment to the environment. To preserve human health and safety, monitoring and managing CO2 levels in the environment is desired. Net zero emissions is the desired outcome for minimizing the worst effects of climate change and limiting the rise in global temperatures. The term “net zero emissions” describes a situation in which the amount of greenhouse gas emissions created is equal to the amount of those emissions that are removed from the atmosphere. Approaches to lower greenhouse emissions include utilization of renewable energy sources, improvements in energy efficiency, and the transition to low-carbon modes of transportation. The above strategies can also be combined with the removal of carbon dioxide from the atmosphere through practices such as reforestation and afforestation, as well as carbon capture, storage, and utilization. Reusing CO2 as a feedstock for various carbon-based compounds, such as methane, formic acid, alcohol, and hydrocarbons, is an alternate approach for removing greenhouse gases from the environment. Moreover, producing syngas by the CO2 reduction reaction (CO2RR) method can address the above issues, while reducing the greenhouse effect.
Electrochemical CO2 reduction is an alternative method for lowering greenhouse gas emissions and creating useful products from CO2 due to its high selectivity, durability, energy efficiency, and versatility. A number of improvements are to be made before widespread industrial use can occur, including raising the effectiveness and longevity of catalysts, enhancing reaction conditions, and lowering overall costs.
Metal-organic frameworks (MOFs) are crystalline, porous materials made up of metal ions or nodes joined by organic linkers. They possess favorable properties, including high porosity, substantial surface area, variable pore size, exceptional tunability, and good stability. A number of applications, such as gas storage, gas separation, catalysis, drug delivery, and sensing, take advantage of the distinctive characteristics of MOFs. One such application is CO2 conversion—an energetically taxing step in the effort to lower the emissions of greenhouse gases. In CO2 conversion operations, MOFs can be utilized as catalysts to transform CO2 into usable products or fuels. The zeolitic imidazolate framework (ZIF) is a subclass of MOFs, with the majority of series including zinc or cobalt as the metal core and imidazole as linkers. Among ZIFs, the Zn-based zeolitic imidazole framework (ZIF-8) shows strong thermal and chemical stability that separates it from other MOFs.
Various ZIF-based catalysts for syngas production have been explored in the past; however, there still exists a need to develop a simple and efficient catalyst for the conversion of carbon dioxide to syngas with high selectivity and efficiency. Accordingly, an object of the present disclosure is directed toward a method for producing syngas with a silver-doped zeolitic imidazolate framework (Ag@ZIF-8)-based electrocatalyst to overcome the limitations of the art.
In an exemplary embodiment, a method for syngas production is described. The method includes contacting an electrocatalytic working electrode with a gaseous electrolytic solution including carbon dioxide in a cell with a reference electrode and a counter electrode. The electrocatalytic working electrode includes an electrocatalytic material on a conductive carbon paper. The electrocatalytic material includes a zeolitic imidazolate framework-8 (ZIF-8) and silver in an amount of 5 to 10 percent by weight of the electrocatalytic material. The ZIF-8 and silver are in a layer of particles. The particles are in the form of one or more polyhedrons. The one or more polyhedrons have a longest dimension with an average length of 0.1 to 20 μm. The electrocatalytic working electrode, the reference electrode, and the counter electrode are in connection with a potentiostat workstation. Applying a potential to the cell during the contacting. Forming a gaseous syngas product by electrocatalytically reducing the carbon dioxide in the gaseous electrolytic solution without forming any liquid syngas product.
In some embodiments, the electrocatalytic material was made by a process that includes forming a first mixture by mixing a silver salt with a first organic solvent. The process includes adding the ZIF-8 to the first mixture. The process further includes sonicating and mixing the first mixture to form a solid, followed by washing the solid. Then, the process includes suspending the solid in a second organic solvent and an acid to form a second solution, and then filtering the second solution to obtain the solid. The process further includes drying the solid to form the electrocatalytic material.
In some embodiments, the electrocatalytic working electrode was made by a process that includes dispersing the electrocatalytic material in an organic solvent, water, and a polymer to form a suspension. The process includes sonicating the suspension. The process further includes drop-casting the suspension onto the conductive carbon paper, and further drying the suspension-covered conductive carbon paper to form the electrocatalytic working electrode.
In some embodiments, the electrocatalytic material is in the form of dodecahedron crystals.
In some embodiments, the dodecahedron crystals have an average length of 100 to 10000 nanometers (nm).
In some embodiments, the electrocatalytic material has a surface area of 1000 to 1300 meters square per gram (m2 g−1).
In some embodiments, the electrocatalytic material has a Tafel value of 150 to 230 millivolts per decade (mV dec−1).
In some embodiments, the electrocatalytic material has a double-layer capacitance of 1.0 to 1.3 millifarads (mF).
In some embodiments, the electrocatalytic material has a charge transfer resistance of 130 to 150 ohms (2).
In some embodiments, the electrocatalytic material has a chronoamperometry stability for 15 to 20 hours in a CO2-saturated 0.1 molar (M) KHCO3 solution.
In some embodiments, the syngas product includes carbon monoxide and hydrogen in a weight ratio of carbon monoxide to hydrogen from 15:85 to 80:20 at a potential of −0.5 to −1.5 V vs. RHE.
In some embodiments, the cell is a flow cell, the electrocatalytic material includes silver in the amount of 10 percent by weight of the electrocatalytic material, the potential is −0.7 V vs. RHE, and the syngas product includes 70% by weight carbon monoxide and 30% by weight hydrogen.
In some embodiments, the cell is a flow cell, the electrocatalytic material includes silver in the amount of 10 percent by weight of the electrocatalytic material, the potential is −0.9 V vs. RHE, and the syngas product includes 80% by weight carbon monoxide and 20% by weight hydrogen.
In some embodiments, the electrocatalytic material has a current density of 25 to 30 milliamperes per square centimeter (mA cm−2).
In some embodiments, the cell is a flow cell, the electrocatalytic material includes silver in the amount of 10 percent by weight of the electrocatalytic material, the potential is −1.1 V vs. RHE, and the syngas product includes 65% by weight carbon monoxide and 35% by weight hydrogen.
In some embodiments, the cell is a flow cell, the electrocatalytic material includes silver in the amount of 10 percent by weight of the electrocatalytic material, the potential is −1.3 V vs. RHE, and the syngas product includes 40% by weight carbon monoxide and 60% by weight hydrogen.
In some embodiments, the cell is an H-cell, the potential is −1.1 V vs. RHE, the syngas product includes 70% by weight carbon monoxide and 30% by weight hydrogen, and the electrocatalytic material has a current density of 2 to 3 mA cm−2.
In some embodiments, the reference electrode is a silver/silver chloride (Ag/AgCl) electrode.
In some embodiments, the counter electrode is a platinum mesh electrode.
In some embodiments, the gaseous electrolytic solution further includes a potassium salt.
These and other aspects of the non-limiting embodiments of the present disclosure will become apparent to those skilled in the art upon review of the following description of specific non-limiting embodiments of the disclosure in conjunction with the accompanying drawings. 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.
A more complete appreciation of this disclosure (including alternatives and/or variations thereof) 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 following description, it is understood that other embodiments may be utilized, and structural and operational changes may be made without departure from the scope of the present embodiments disclosed herein.
When describing the present disclosure, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise. Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings wherever applicable in which some, but not all, embodiments of the disclosure are shown.
Reference will now be made to specific embodiments or features, examples of which are illustrated in the accompanying drawings. In the drawings, whenever possible, corresponding or similar reference numerals will be used to designate identical or corresponding parts throughout the several views. Moreover, references to various elements described herein are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be constructed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claims. In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the use of singular includes plural and the words “a,” “an,” and the like generally carry a meaning of “one or more” and “at least one,” unless otherwise stated.
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.
The use of the terms “include,” “includes,” “including,” “have,” “has,” or “having” should be generally understood as open-ended and non-limiting unless specifically stated otherwise.
As used herein, the term “electrode” refers to an electrical conductor used to make contact a non-metallic part of a circuit (e.g., a semiconductor, an electrolyte, a vacuum, or air). An electrode is a point where an electric current enters and leaves the electrolyte. An electrode may be an anode or a cathode. An anode is the electrode through which the electric current enters and a cathode is the electrode through which the electric current leaves. Electrodes transport produced electrons from one half-cell to another, which produces an electrical charge.
As used herein, the term “current density” refers to the amount of electric current traveling per unit area of a chosen cross-section.
As used herein, the term “Tafel slope” refers to the relationship between the overpotential and the logarithmic current density.
As used herein, the term “electrochemical cell” refers to a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. Electrochemical cells which generate electrical energy are voltaic cells or galvanic cells. Electrochemical cells which use electrical energy are electrolytic cells. Electrochemical cells comprise two half-cells. The two half-cells comprise separate oxidation reactions and reduction reactions.
As used herein, the term “overpotential” refers to the difference between the applied potential at the working electrode and the equilibrium potential of the net redox reaction.
Overpotential 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 to drive a reaction as compared to that which is thermodynamically thought 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 can be experimentally measured by determining the potential at which a given current density is reached.
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 toward the use of Ag and ZIF-8 composite materials for the electrocatalytic reduction of CO2 (CO2RR). ZIF-8 is used to anchor doped silver nanoparticles by a simple, low-temperature, chemical deposition technique. Characterization and application of the resulting catalysts for electrochemical (CO2) reduction reaction (ECO2RR) to syngas is evaluated in aqueous solutions of 0.1 M KHCO3 at room temperature in an H-cell. In addition, the materials are examined in flow cells to assess their electrocatalytic properties in depth to produce pure syngas at various potentials. The results indicate that CO2 electroreduction on Ag-doped ZIF-8 catalysts produces just CO and H2, without having any liquid fuel, resulting in a total faradaic efficiency approaching about 100%. The ratio of H2 to CO (H2/CO) is adjustable from 3:1 to 1:3 by changing an applied voltage during the electrochemical CO2 reduction reaction.
At step 72, the method 70 includes contacting an electrocatalytic working electrode with a gaseous electrolytic solution including carbon dioxide in a cell with a reference electrode and a counter electrode. In some embodiments, the gaseous electrolytic solution includes a potassium salt. As used herein, the term “reference electrode” refers to an electrode with a stable and well-known electrode potential. In some embodiments, the reference electrode is a silver/silver chloride (Ag/AgCl) electrode. In some embodiments, the Ag/AgCl electrode may be filled with potassium chloride (KCl). The KCl in the Ag/AgCl may have a saturated concentration, a concentration of about 3.0 molal, or a concentration of about 3.0 molar. In a preferred embodiment, the Ag/AgCl electrode has a saturated concentration of KCl. Further, as used herein, the term “counter-electrode” refers to the electrode used in an electrochemical cell for voltammetric analysis or other reactions (e.g., CO2 reduction) in which an electric current is thought to flow. The counter electrode, also known as an auxiliary electrode, closes the electric current circuit in the electrochemical cell. An outer surface of the counter electrode may include an inert, electrically conducting chemical substance, such as platinum, gold, or carbon. The carbon may be in the form of graphite or glassy carbon. In one embodiment, the counter electrode may be a wire, a rod, a cylinder, a tube, a scroll, a sheet, a piece of foil, a woven mesh, a perforated sheet, or a brush. In some embodiments, the counter electrode is a platinum mesh electrode.
The counter electrode material should thus be sufficiently inert to withstand the chemical conditions in the electrolyte solution, such as acidic or basic pH values, without substantially degrading during the electrochemical reaction. In addition, the counter electrode should preferably not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable electrode contamination. In some embodiments, the reference electrode and the counter-electrode may be connected through electrical interconnects that allow for the passage of current between the electrodes when a potential is applied between them. In an embodiment, the reference electrode and the counter-electrode can have the same or different dimensions. The reference electrode and the counter-electrode may be arranged as obvious to a person of ordinary skill in the art. The electrocatalytic working electrode, the reference electrode, and the counter electrode are in connection with a potentiostat workstation.
The electrocatalytic working electrode includes an electrocatalytic material on a conductive carbon paper. The electrocatalytic material includes a zeolitic imidazolate framework-8 (ZIF-8) and silver in an amount of 5 to 10 percent by weight of the electrocatalytic material. The ZIF-8 may be a porous material.
The ZIF-8 and silver are in a layer of particles. The layer of particles is on the conductive carbon paper. In some embodiments, the layer of particles may be attached directly to the conductive carbon paper. In other embodiments, the layer of particles may be attached to the conductive carbon paper with a polymer, a chemical binder, a physical binder, or other means of attachment known in the art. The layer of particles may have a thickness of 0.1 to 1000 μm, preferably 1 to 500 μm, preferably 10 to 200 μm, and more preferably 20 to 100 μm.
The particles are in the form of one or more polyhedrons. The one or more polyhedrons may be any three-dimensional shape with flat polygonal faces, straight edges, and sharp corners or vertices. In some embodiments, the one or more polyhedrons may be cubes, prisms, pyramids, tetrahedrons, octahedrons, dodecahedrons, icosahedrons, any polyhedrons known in the art, and a combination thereof. In a preferred embodiment, the one or more polyhedrons are dodecahedrons. The one or more polyhedrons may be of varying sizes and shapes. The one or more polyhedrons may be attached to one or more other polyhedrons. The one or more polyhedrons may have a longest dimension with an average length of 100 to 10000 nm, preferably 200 to 5000 nm, more preferably 500 to 3000 nm, and yet more preferably 700 to 2000 nm. In a preferred embodiment, the one or more polyhedrons have a longest dimension with an average length of 0.1 to 20 μm.
In some embodiments, the silver is in the form of nanoparticles. In an embodiment, the silver is in the form of nanoparticles having an average diameter of 1 to 20 nm, preferably 2 to 15 nm, more preferably 4 to 10 nm, and yet more preferably 5 to 8 nm. In other embodiments, the silver is in the form of flakes, flecks, sheets, nanosheets, particles, the like, and a combination thereof. In some embodiments, the silver is dispersed on the surface of the ZIF-8. In some embodiments, the silver may be dispersed on 1 to 100%, preferably 5 to 80%, preferably 10 to 60%, preferably 20 to 50%, or preferably 30 to 40% of the surface of the ZIF-8. The silver is not agglomerated. In some embodiments, the silver is dispersed within pores of the ZIF-8. In some embodiments, the silver may be dispersed within 5 to 100%, preferably 10 to 80%, preferably 20 to 60, or preferably 30 to 50% of a total pore amount of the ZIF-8. In some embodiments, the silver dispersed within pores of the ZIF-8 may fill the pores of the ZIF-8 partially or completely. In some embodiments, the silver may fill 5 to 100%, preferably 10 to 80%, preferably 20 to 60, or preferably 30 to 50% of the pores of the ZIF-8. In some embodiments, the silver is dispersed with a framework of the ZIF-8. In some embodiments, the silver may have an oxidation state of 0, +1, or a combination thereof.
In some embodiments, the electrocatalytic material is in the form of dodecahedron crystals. In some embodiments, the dodecahedron crystals have a longest dimension with an average length of 100 to 10000 nanometers (nm), preferably 200 to 5000 nm, more preferably 500 to 3000 nm, and yet more preferably 700 to 2000 nm. In some embodiments, the dodecahedron crystals have an average length of about 100 nm, about 200 nm, about 300 nm, about 400 nm, about 500 nm, about 600 nm, about 700 nm, about 800 nm, about 900 nm, about 1000 nm, about 2000 nm, about 3000 nm, about 4000 nm, about 5000 nm, about 6000 nm, about 7000 nm, about 8000 nm, and about 9000 nm. In some embodiments, the electrocatalytic material comprises dodecahedron crystals of varying lengths.
In some embodiments, the electrocatalytic material has a surface area of 1000 to 1300 square meters per gram (m2 g−1), preferably 1050 to 1250 m2 g−1, preferably 1100 to 1200 m2 g−1, or preferably about 1250 m2 g−1. In some embodiments, the electrocatalytic material has a surface area of about 1000 m2 g−1, about 1100 m2 g−1, about 1200 m2 g−1, and about 1300 m2 g−1. In some embodiments, the electrocatalytic material has a Tafel value of 150 to 230 millivolts per decade (mV dec−1), preferably 150 to 220 mV dec−1, preferably 170 to 200 mV dec−1, or preferably 180 to 190 mV dec−1. In some embodiments, the electrocatalytic material has a Tafel value of about 150 mV dec−1, about 160 mV dec−1, about 170 mV dec−1, about 180 mV dec−1, about 190 mV dec−1, about 200 mV dec−1, about 210 mV dec−1, about 220 mV dec−1, and about 230 mV dec−1.
In some embodiments, the electrocatalytic material has a double layer capacitance of 1.0 to 1.3 millifarads (mF), preferably 1.1 to 1.2 mF, or preferably about 1.15 mF. In some embodiments, the electrocatalytic material has a double layer capacitance of about 1.0 mF, about 1.1 mF, about 1.2 mF, about 1.25 mF, and about 1.3 mF.
In some embodiments, the electrocatalytic material has a charge transfer resistance of 130 to 150 ohms (Ω), preferably 135 to 145Ω, or preferably about 140Ω. In some embodiments, the electrocatalytic material has a charge transfer resistance of about 130Ω, about 131Ω, about 132Ω, about 133Ω, about 134Ω, about 135Ω, about 136Ω, about 137Ω, about 138Ω, about 139Ω, about 140Ω, about 141Ω, about 142Ω, about 143Ω, about 144Ω, about 145Ω, about 146Ω, about 147Ω, about 148Ω, about 149Ω, and about 150Ω.
In some embodiments, the electrocatalytic material has a chronoamperometry stability for 15 to 20 hours, preferably 16 to 19 hours, or preferably 17 to 18 hours, in a CO2-saturated 0.1 molar (M) KHCO3 solution. In some embodiments, the electrocatalytic material has a chronoamperometry stability for about 15 hours, about 16 hours, about 17 hours, about 18 hours, about 19 hours, and about 20 hours in a CO2-saturated 0.1 M KHCO3 solution.
In some embodiments, the electrocatalytic material has a current density of 25 to 30 milliamperes per square centimeter (mA cm−2), preferably 26 to 29 mA cm−2, or preferably 27 to 28 mA cm−2. In some embodiments, the electrocatalytic material has a current density of about 25 mA cm−2, about 26 mA cm−2, about 27 mA cm−2, about 28 mA cm−2, about 29 mA cm−2, and about 30 mA cm−2.
In an embodiment, the electrocatalytic working electrode is made by a process that includes dispersing the electrocatalytic material in an organic solvent, water, and a polymer to form a suspension. In some embodiments, the water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In one embodiment, the water is bidistilled to eliminate trace metals. In some embodiments, suitable polymers may include, for example, poly(vinylidene) fluoride (PVDF), poly(tetrafluoroethylene) (PTFE), a substituted poly(tetrafluoroethylene), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(methacrylic acid) (PMAA), poly(acrylic acid) (PAA), poly(vinyl methyl ketone), and poly(ethylene terephthalate) (PET), polysulfone (PSf), polyethersulfone (PES), poly(ether sulfone) (PSF), polyacrylonitrile (PAN), polypropylene (PP), polyimide (PI), and poly(arylene ether nitrile ketone) (PPENK), which can used alone or in combination. In an embodiment, the polymer is a substituted poly(tetrafluoroethylene). In an embodiment, the substituted poly(tetrafluoroethylene) may be substituted with a hydroxyl group, a carbonyl group, a ketone, an amine, an amide, a phosphate group, a nitrate group, an oxide, a sulfate group, and the like. In a preferred embodiment, the polymer is Nafion. The process includes sonicating the suspension. The process includes drop-casting the suspension onto the conductive carbon paper. In some embodiments, the casting may be substituted by spraying, electrostatic spraying, spin casting, spin coating, dipping, painting, dripping, brushing, immersing, flowing, exposing, electrostatic spraying, pouring, rolling, curtaining, wiping, printing, pipetting, and the like. The process includes drying the suspension-covered conductive carbon paper to form the electrocatalytic working electrode.
At step 74, the method 70 includes applying a potential to the cell during the contacting. In an embodiment, the syngas product comprises a ratio of carbon monoxide and hydrogen produced in the reduction of carbon dioxide. In an embodiment, the ratio of carbon monoxide and hydrogen may be tunable by altering one or more conditions of the contacting, changing an applied potential, a combination thereof, and the like. In some embodiments, the syngas product includes carbon monoxide and hydrogen in a weight ratio of carbon monoxide to hydrogen from 15:85 to 80:20 at a potential of −0.5 to −1.5 V vs. RHE. In an embodiment, the cell is a flow cell, the electrocatalytic material includes silver in the amount of 10 percent by weight of the electrocatalytic material, the potential is −1.1 V vs. RHE, and the syngas product includes about 65% by weight carbon monoxide and about 35% by weight hydrogen. In an embodiment, the cell is a flow cell, the electrocatalytic material includes silver in the amount of 10 percent by weight of the electrocatalytic material, the potential is −1.3 V vs. RHE, and the syngas product includes about 40% by weight carbon monoxide and about 60% by weight hydrogen. In an embodiment, the cell is an H-cell, the potential is −1.1 V vs. RHE, the syngas product includes about 70% by weight carbon monoxide and about 30% by weight hydrogen, and the electrocatalytic material has a current density of 2 to 3 mA cm−2.
At step 76, the method 70 includes forming a gaseous syngas product by electrocatalytically reducing the carbon dioxide in the gaseous electrolytic solution without forming any liquid syngas product. In a specific embodiment, the CO2 electroreduction on Ag-doped ZIF-8 catalysts produces just CO and H2, without having any liquid fuel, resulting in a total faradaic efficiency approaching about 100%.
At step 82, the process 80 includes dissolving a silver salt in an organic solvent to form a first solution. Suitable examples of the silver salt include silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), silver nitrate (AgNO3), silver nitrite (AgNO2), and/or combinations thereof. In a preferred embodiment, the silver salt is silver nitrate. In some embodiments, the organic solvent may include, but is not limited to, tetrahydrofuran, toluene, ethyl acetate, dichloromethane, dimethylformamide, acetonitrile, acetone, benzene, cyclohexane, diethyl ether, dimethyl sulfoxide, nitromethane, propylene carbonate, ethanol, formic acid, n-butanol, methanol, the like, and/or any combination thereof. In a preferred embodiment, the organic solvent is methanol. The weight-by-volume ratio of the silver salt to the organic solvent is in a range of 1:1 to 1:5, preferably 1:2 to 1:4, and more preferably about 1:2.
At step 84, the process 80 includes adding the ZIF-8 to the first solution. In an embodiment, the weight ratio of the silver salt to the ZIF-8 is in the range of 1:5 to 1:15, preferably 1:6 to 1:14, preferably 1:7 to 1:13, preferably 1:8 to 1:12, preferably 1:9 to 1:11, and more preferably about 1:10.
At step 86, the process 80 includes sonicating and mixing the first solution to form a solid. As used herein, the term “sonication” refers to the process in which sound energy and sound waves are used to agitate particles in a solution. Sonication may be referred to as ultrasonication. Sonication may occur with a probe, a bath, a basket, like, and a combination thereof. In some embodiments, other modes of agitation known to those of ordinary skill in the art, for example, via stirring, swirling, mixing, or a combination thereof may be employed to form the resultant mixture. In an embodiment, the sonication was carried out for a period of 5-15 minutes, preferably 6-14 minutes, preferably 7-13 minutes, preferably 8-12 minutes, preferably 9-11 minutes, and more preferably about 10 minutes. In an embodiment, the mixing occurs for 1-3 hours, preferably about 2 hours.
At step 88, the process 80 includes washing the solid. The solid may be washed with ethanol and water. The washing may be carried out 1 to 5 times, preferably 2 to 4 times, and more preferably about 3 times. The washing may be carried out to remove any unreacted silver ions in the solid.
At step 90, the process 80 includes suspending the solid in an organic solvent and an acid to form a second solution. In some embodiments, the organic solvent may include, but is not limited to, tetrahydrofuran, toluene, ethyl acetate, dichloromethane, dimethylformamide, acetonitrile, acetone, benzene, cyclohexane, diethyl ether, dimethyl sulfoxide, nitromethane, propylene carbonate, ethanol, formic acid, n-butanol, methanol, the like, and/or any combination thereof. In a preferred embodiment, the organic solvent is methanol. In some embodiments, the acid may include formic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, phosphoric acid, sulfuric acid, acetic acid, palergonic acid, ascorbic acid, the like, and a combination thereof. In a preferred embodiment, the acid is an ascorbic acid.
At step 92, the process 80 includes filtering the second solution to obtain the solid. Filtration is the process of separating suspended solid matter from a liquid, by causing the liquid to pass through a porous material, such as a filter paper. Filtering may occur with a glass funnel, a Buchner funnel, a Hirsch funnel, a glass frit, and the like. Filtering may be done by gravity filtration, vacuum filtration, and the like.
At step 94, the process 80 includes drying the solid to form the electrocatalytic material. The solid can be dried by using heating appliances such as ovens, microwaves, autoclaves, hot plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, and hot-air guns. The drying is carried out at a temperature range of 30-60° C., preferably 40-55° C., and more preferably at about 50° C., for 2-8 hours, preferably 3-7 hours, preferably 4-6 hours, more preferably for about 5 hours to obtain the electrocatalytic material. In a preferred embodiment, the drying may be carried out under a vacuum to prevent any further oxidation of the electrocatalytic material.
The following examples demonstrate a method for syngas using a silver-doped zeolitic imidazolate framework (Ag@ZIF-8, Ag-ZIF-8)-based electrocatalyst. 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.
Zinc nitrate (ZnNO3) (99.95%), silver nitrate (AgNO3), ascorbic acid (99.0%), 2-methyl imidazole (99.0%), potassium bicarbonate (99.9%), and potassium hydroxide (99.5%) were purchased from Sigma Aldrich, St. Louis, MO, USA. Methanol (CH3OH) (99.8%) was procured from Sharlu (Sharjah, United Arab Emirates).
ZIF-8 was prepared by dissolving 1.31 grams (g) of Zn(NO3) 2 in 45 milliliters (mL) of methanol in a 100 ml beaker. In another beaker, 2.87 g of 2-methyl imidazole was dissolved until a clear solution appeared. The two clear solutions were mixed in a 150 mL round bottom flask and stirred at room temperature for 1 hour. The synthesized white ZIF-8 suspension was separated by centrifugation at 8000 rotations per minute (rpm) for 10 minutes. The white crystal was washed three times with methanol.
The 5% Ag-ZIF-8 was prepared as follows (as shown in
Ten milligrams of the ZIF-8 or Ag-ZIF-8 catalyst was dispersed in 1 mL mixture of 750 microliters (μL) isopropanol, 200 μL DI water, and 50 μL Nafion (5%). The mixture was sonicated for 20 minutes. Then, 100 μL of the suspension was drop-cast onto 1 square centimeter (cm2) conductive carbon paper and dried at room temperature.
The spray painting method was applied to prepare the working electrodes for the flow cell. This process is efficient and cost-effective for creating gas diffusion electrode (GDE) for electrochemical performance. 20 milligrams of the ZIF-8 or Ag-ZIF-8 catalyst was dispersed in a 2 mL mixture of 1500 μL isopropanol, 400 μL DI water, and 100 μL Nafion (5%). The mixture was homogenized using a magnetic stirrer or ultrasonication. Next, 200 μL of the prepared mixture was kept under constant air flow and pressure and sprayed on GDE using the spray gun method. Using a high-pressure air spray gun, the catalyst mixture was sprayed onto the carbon support material surface. The spray nozzle is operated at 1.5 to 3 bar of pressure, with a 2 to 5 cm gap between the spray nozzle and the carbon-based gas diffusion electrode (GDE). The GDE coated with the catalyst mixture was then dried for 30 to 60 minutes at a temperature of 60 to 80° C. before its application in the flow cell.
Morphological and detailed microstructural attributes of the materials were discerned by field emission scanning electron microscopy (FESEM, Tescan Lyra-3, Kohoutovice, Czech Republic). The sample was gold-coated for 30 seconds before SEM and Energy-Dispersive X-ray spectroscopy (EDS) analysis. Another technique employed for the characterization of the samples was X-ray diffraction (XRD, Rigaku MiniFlex, Austin, TX, USA) to reveal the crystal structure of the materials. To begin, the samples were finely powdered using a mill and pestle to ensure consistency and remove any big pieces. The next step was to put the powdered samples into a sample holder. The sample was then prepared for analysis by having its surface flattened and leveled using a glass slide. This helps to ensure that reliable results are obtained. After that, the sample holder was transferred into the XRD machine, and the analysis commenced. The obtained XRD was matched with the simulated XRD of the materials. Fourier-transform infrared (FTIR, Thermo, Waltham, MA, USA) was used to identify the functional groups in the materials. Brunauer-Emmett-Teller (BET) surface analyzer (Triplex) was used to calculate the porous nature of a material. This method is based on gas adsorption as a function of pressure. Typically the sample is first degassed at 120° C. for 6 hours. Under a nitrogen environment, the gas pressure is measured vs. the amount of gas adsorbed by the sample to calculate the porosity. A gas chromatographer (GC) equipped with barrier ion discharge detector (Shimadzu, Kyoto, Japan) and potentiostat (Gammray 620, Warminster, UK) were used.
The ECO2RR performance was investigated with the aid of H-cell and flow cell systems. The H-cell consisted of a silver silver-chloride electrode (Ag/AgCl) as a reference electrode. A platinum mesh was used as a counter electrode. As-prepared Ag@ZIF-8 film on conductive carbon paper was used as the working electrode. A potentiostat (Gammray 620) was connected to the electrodes in the cell.
The flow cell setup consisted of three main components. The first component was the electrolyte compartments, with one compartment containing the catholyte, which was 0.5 M KHCO3, and the second compartment containing the anolyte, which was 1.0 M KOH. The second component was the cell, which consisted of the cathode part (where the CO2 gas passes on one side of the GDE and the catholyte passes on the other side) and the anode part is connected to the anolyte. The two parts of the cell were separated by a proton permeable membrane to allow the produced protons (H+) to pass from the anode to the cathode. The third component of the flow cell was the pump, which controlled flows and circulated the catholyte and anolyte between the electrolyte compartments of the cell. Similar to the H-cell, the reference electrode was connected with a working electrode (GDE) on the cathode side, and a counter electrode on the anode side, and all were connected to a potentiostat workstation (Gammray 620).
The ECO2RR performance was evaluated by carrying out linear sweep voltammetry (LSV) techniques and calculation of the overpotential at different current densities (current normalized to the geometric surface area of the electrode). The cyclic voltammetry (CV) and LSV experiments were performed in 0.1 M potassium bicarbonate (KHCO3).
where EAg/AgCl=0.199 volts (V).
The potential was swept from (0.0 to −1.4 V vs. RHE). The electrochemical impedance spectroscopy (EIS) was performed by varying the frequency from 105 to 0.1 hertz (Hz) under identical electrolyte and electrodes to the LSV.
ZIF-8 contains zinc atoms as metal centers, which are considered promising active sites for CO2 electroreduction to CO. In addition, the ZIF-8 contains homogenous pores that can act as nano-reactors to confine the growth of Ag+ nanoparticles. Finally, a mild reducing agent such as ascorbic acid is used to reduce Ag+ into metallic Ag nanoparticles within the pores of the ZIF-8. The washing step prior to reduction was used to remove 50 to 100%, preferably 60 to 100%, preferably 70 to 100%, preferably 80 to 100%, or preferably 90 to 100% of the surface Ag ions to avoid agglomeration and the growth of large Ag particles, which could lead to a drop in the surface area. The as-prepared Ag-ZIF-8 was characterized by several techniques.
XRD was carried out to investigate the phase formation and purity of the prepared material. As can be observed in
The morphological and structural properties of the prepared material were investigated by SEM.
The surface area of the ZIF-8 and Ag-ZIF-8 was investigated with the aid of the BET surface analyzer (
In order to demonstrate the overall electrocatalytic performance, the electrocatalysts (ZIF-8, 5% Ag-ZIF-8, 10% Ag-ZIF-8) were analyzed using linear sweep voltammetry (LSV) in N2- or CO2-saturated 0.1 M KHCO3 electrolytes (
To gain insight about the mechanism and the kinetics of the ECO2RR, the Tafel slope was estimated from the polarization curves (
Another factor is the electrochemical active surface area (ESCA), which can be estimated from the double-layer capacitance (Cdl). The Cal was evaluated by recording cyclic voltammograms (
The electrode durability was investigated using chronoamperometry (
The determined electrode (10% Ag-ZIF-8) in the H-cell was also investigated in the flow cell system, which is a more practical setup. The LSV and FE obtained from the flow cell were compared with the results obtained from the H-cell. As can be observed in
The electrochemical performance and conversion efficiency of the current findings are shown in Table 1, which compares Ag-based composites, ZIF-8 composites, and Ag-ZIF-8 composites for the electroreduction of CO2 into usable liquid chemicals. The 10% Ag-ZIF-8 produced syngas with a FE ratio of 70:30 (CO:H2) at a potential of −1.1 V vs. RHE and 80:20 at a potential of −0.9 V vs. RHE, which is an improvement over previous reports.
‘a’ corresponds to Wang, Y.; Hou, P.; Wang, Z.; Kang, P. Zinc Imidazolate Metal-Organic Frameworks (ZIF-8) for Electrochemical Reduction of CO2 to CO. Chemphyschem. 2017, 18, 3142-3147, incorporated herein by reference in its entirety; ‘b’ corresponds to Jiang, X.; Li, H.; Gao, D.; Si, R.; Yang, F.; Li, Y.; Wang, G.; Bao, X. Carbon dioxide electroreduction over imidazolate ligands coordinated with Zn(II) center in ZIFs. Nano Energy. 2017, 52, 345-350, incorporated herein by reference in its entirety; ‘c’ Dou, S.; Song, J.; Xi, S.; Du, Y.; Wang, J.; Huang, Z.-F.; Xu, Z. J.; Wang, X. Electrochemical CO2 Reduction on Metal-Organic Frameworks via Ligand Doping. Angew. Chem. Int. Ed. 2019, 58, 4041-4045, incorporated herein by reference in its entirety; ‘d’ corresponds to Jiang, X.; Wu, H.; Chang, S.; Si, R.; Miao, S.; Huang, W.; Li, Y.; Wang, G.; Bao, X. Boosting CO2 electroreduction over layered zeolitic imidazolate frameworks decorated with Ag2O nanoparticles. J. Mater. Chem. A. 2017, 5, 19371-19377, incorporated herein by reference in its entirety; ‘e’ corresponds to Yan, S.; Chen, C.; Zhang, F.; Mahyoub, S. A.; Cheng, Z. High-density Ag nanosheets for selective electrochemical CO2 reduction to CO. Nanotechnology, 2021, 32, 165705, incorporated herein by reference in its entirety; ‘f’ corresponds to Kim, B.; Ma, S.; Molly Jhong, H.-R.; Kenis, P. J. A. Influence of dilute feed and pH on electrochemical reduction of CO2 to CO on Ag in a continuous flow electrolyzer. Electrochim. Acta. 2015, 166, 271-276, incorporated herein by reference in its entirety; ‘g’ corresponds to Li, Y. C.; Zhou, D.; Yan, Z.; Gonçalves, R. H.; Salvatore, D. A.; Berlinguette, C. P.; Mallouk, T. E. Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells. ACS Energy Lett. 2016, 1, 1149-1153, incorporated herein by reference in its entirety; and ‘h’ corresponses to work of the present disclosure. In the present disclosure, silver-doped ZIF-8 (Ag@ZIF-8) that acted as a catalyst for the production of syngas (CO and H2) at various loading and at various potentials is prepared. The experimental findings show that Ag-doped ZIF materials have a higher current density than ZIF-8. Furthermore, the flow cell has a higher current density than the H-cell. The product analysis revealed near 100% FE for the gas products. Based on qualitative and quantitative analyses, the products contained syngas at various ratios of H2 and CO, and could be influenced by the applied potential. These findings reveal that the Ag-ZIF-8 platform offers promising materials for effective CO2 conversion to syngas.
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 disclosure may be practiced otherwise than as specifically described herein.
