Patent Application
20240426005
Support provided by the Center for Refining and Advanced Chemicals, Research Institute at King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia, is gratefully acknowledged.
The present disclosure is directed to an electrode, and particularly to a lead electrode with improved selectivity for electrochemical CO2 reduction reaction (CO2RR).
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 it is 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 invention.
Several fossil fuels, including petroleum, coal, and natural gas, have long been relied upon as energy sources for sustaining economic activities. However, extensive utilization of such fuels has continued to provoke global concerns due to the continuous emission of CO2 into the atmosphere. This ongoing release of CO2 contributes to adverse climatic and environmental effects. At present, the atmospheric concentration of CO2 has exceeded 410 parts per million (ppm), and this has prompted global research into finding sustainable solutions to mitigate the detrimental impacts caused by excessive CO2 levels on our planet.
The electrocatalytic conversion of CO2 into valuable chemicals and fuels is a promising approach to mitigate the atmospheric CO2 concentration. This conversion process can be carried out under ambient conditions and can be powered by renewable energy sources like solar and wind energies. Additionally, this technology provides an efficient means of energy storage in the form of fuels, chemicals, and essential industrial feedstocks such as CH4, CO, C2H4, and formate. However, despite its numerous advantages, there are certain challenges that need to be addressed for practical implementation. One of the key challenges is the lack of durable electrocatalysts and the poor selectivity of existing catalysts. Furthermore, the competing hydrogen evolution reaction (HER) poses an obstacle to the efficiency of the process.
To overcome these challenges, the design of highly selective catalysts that can target specific products is desired. These catalysts should exhibit low CO2 activation overpotentials, enabling effective conversion while minimizing energy requirements. Additionally, such catalysts would eliminate the need for costly product separation processes, further enhancing the economic feasibility of electrocatalytic CO2 conversion.
Metal oxide-based electrocatalysts, such as Pb, Sn, Ni, Cu, and Pt have shown the ability to reduce CO2 to formate, CO, and alcohols. A number of these catalysts have been synthesized through the deposition of metal nanoparticles on active supports, the decomposition of the metal precursors onto the catalysts support, and/or the solvent-assisted synthesis method. The solvent-assisted synthesis method often enables the design of electrocatalysts with controlled morphology and size, however, this method often generates a significant amount of organic waste, posing environmental concerns and making the overall process environmentally unsafe. Hence, there is a need to develop a method of making the electrocatalysts that may remove or eliminate the aforementioned limitations.
In view of the foregoing, it is one objective of the present disclosure to describe a Pb-based electrode with an enhanced-performance. A second objective of the present disclosure is to describe a method of making the Pb-based electrode. A third objective of the present disclosure is to describe a method of electrochemical carbon dioxide (CO2) reduction (CO2RR).
In an exemplary embodiment, an electrode is described. The electrode includes a transparent substrate. The electrode includes a lead (Pb) layer at least partially covering a surface of the transparent substrate. In some embodiments, the Pb layer includes irregular octahedral-shaped Pb particles having an average particle size of from 0.5 to 3 micrometers (μm). In some embodiments, the irregular octahedral-shaped Pb particles are uniformly distributed on a surface of the Pb layer.
In some embodiments, the transparent substrate is a glass substrate. The glass substrate is 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 transparent substrate is a glassy carbon substrate.
In some embodiments, the Pb layer has an arithmetic average roughness (Sa) in a range of 2.5 to 3.8 μm.
In some embodiments, the Pb layer has a root mean square roughness (Sq) in a range of 3.0 to 5.7 μm.
In some embodiments, the electrode has a current density of 40 to 100 milliamperes per square centimeter (mA/cm2) at a potential of −0.5 to −0.7 Volt (VAg/AgCl).
In some embodiments, the electrode has a Tafel slope of 200 to 360 millivolts per decade (mV/decade) for CO production in a salt solution at a scan rate of 5 to 50 millivolts per second (mV/s).
In an exemplary embodiment, a method of making the electrode is also described. The method includes disposing a lead sheet on a surface of the transparent substrate to form a Pb-coated substrate having the Pb layer. The method includes immersing the Pb-coated substrate into a solution including a potassium salt and applying a cyclic voltammetry (CV) potential to the Pb-coated substrate to form substantially irregular octahedral-shaped Pb particles on the surface of the Pb layer. In some embodiments, the irregular octahedral-shaped Pb particles have an average particle size of about 1 μm and are uniformly distributed on the surface of the Pb layer. The method further includes removing the Pb-coated substrate from the electrolyte solution, washing, and drying to form the electrode.
In some embodiments, the potassium salt includes potassium chloride (KCl). The KCl is present in the solution at a concentration of 0.001 to 1 molar (M).
In some embodiments, the CV potential is in a range of from −0.5 to −0.7 VAg/AgCl at a scan rate of 10 to 30 mV/s.
In some embodiments, the method includes applying the CV potential for 1 to 100 cycles.
In another exemplary embodiment, a method of electrochemical carbon dioxide (CO2) reduction (CO2RR) is also provided. The method includes charging an electrolyte to an electrochemical cell including a working electrode, a counter electrode, and a reference electrode. The working electrode includes the electrode. The method further includes introducing a CO2—containing gas composition into the electrochemical cell containing the electrolyte and passing the CO2—containing gas composition through the CO2—containing gas composition. The method includes during the passing, simultaneously applying a potential between the working electrode and the counter electrode in the electrochemical cell via the electrolyte to form a product including carbon monoxide (CO) and a formate species. In some embodiments, the formate species is bonded to the surface of the electrode.
In some embodiments, the electrochemical cell is preferably a H-type sealed two-compartment electrolytic cell separated by a proton exchange membrane.
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 CO2—containing gas composition is introduced into the electrochemical cell at a rate of 1 to 50 standard cubic centimeters per minute (sccm).
In some embodiments, the formate species includes at least one of sodium formate, potassium formate, and formic acid. The product further includes hydrogen, ethanol, and methanol.
In some embodiments, the method has a CO faradaic efficiency (FE) of about 89% based on the total charge passed during the applying the potential.
In some embodiments, the method has a CO production rate of from 25 to 35 micromoles per hour per square centimeter (μmol hr−1 cm−2) at an overpotential of −0.8 to −0.6 VRHE.
The foregoing general description of the illustrative present disclosure 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 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, like 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 there between.
As used herein, the term “working electrode” generally refers to the electrode in an electrochemical cell/device/sensor on which the electrochemical reaction of interest is occurring.
As used herein, the term “counter-electrode”, generally refers to an electrode used in an electrochemical cell for voltammetric analysis or other reactions in which an electric current is expected to flow.
As used herein, the term “glassy carbon” generally refers to a non-graphitizing carbon that combines glassy and ceramic properties with those of graphite.
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.
The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.
With the continuous increase in energy consumption and rise in atmospheric CO2 level, electrochemical CO2 reduction reaction (CO2RR), powered by renewable energy sources, provides an approach to convert CO2 to value-added chemicals and achieve a carbon-neutral cycle. However, tuning the selectivity of these catalysts towards the production of other value-added chemicals remains a challenge. Aspects of the present disclosure are directed to a facile approach for tuning the selectivity of Pb electrodes by electrochemical roughening in an electrolyte.
According to an aspect of the present disclosure, an electrode is described. The electrode includes a transparent substrate. In some embodiments, the transparent substrate is a glass substrate. The glass substrate is 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 a preferred embodiment, the transparent substrate is a glassy carbon substrate. Optionally, the glass substrate may be replaced by a titanium plate or a copper plate. The substrate may be of a single layer or several layers. In an example, the substrate made of glass may optionally be coated with a layer of FTO or PTO, or any other materials as stated above, and then sintered together to provide the substrate.
In some embodiments, the substrate may have a thickness of less than or equal to about 3 mm, for example, ranging from about 0.1 mm to about 2.5 mm, from about 0.3 mm to about 2 mm, from about 0.7 mm to about 1.5 mm, or from about 1 mm to about 1.2 mm, including all ranges and subranges therebetween. However, it is preferably 0.5 mm to 3 mm from the viewpoint of easiness in handling. Other ranges are also possible.
The electrode includes a lead (Pb) layer at least partially covering the surface of the transparent substrate. In an embodiment, the surface of the glassy carbon substrate is deposited partially or wholly with at least one layer of the Pb layer in a uniform and continuous manner. In a preferred embodiment, the Pb layer forms a continuous layer on the glassy carbon substrate. In an embodiment, particles of the Pb layer form a monolayer on the glassy carbon substrate. In another embodiment, the particles of the Pb layer may form more than a single layer on the glassy carbon substrate. The deposition of the Pb layer on the glassy carbon substrate results in generating a synergic effect between the Pb layer and the glassy carbon substrate, which enhances the electrical conductivity of the catalyst.
In some embodiments, at least 50% of the surface of the transparent substrate is covered by the Pb layer based on a total surface area of the transparent substrate, preferably at least 70%, preferably at least 90%, or even more preferably at least 99%, based on the total surface area of the transparent substrate. In another embodiment, only one side of the transparent substrate is covered with the Pb layer. Other ranges are also possible.
The Pb layer includes irregular octahedral-shaped Pb particles having an average particle size of from 0.5 to 3 micrometers (μm), preferably 0.1 to 2 μm, preferably 0.3 to 1.8 μm, preferably 0.5 to 1.5 μm, preferably 0.7 to 1.3 μm, preferably 0.9 to 1.1 μm, preferably 1 μm. The irregular octahedral-shaped Pb particles are uniformly distributed on the surface of the Pb layer. The Pb particles may exist in other geometric shapes, such as circular, polygonal, triangular, and rectangular.
As used herein, the term “rough surface” or “rough surface morphology” generally refers to the physical characteristics or features of a surface that deviate from smoothness or regularity. The term “rough surface morphology” may include unevenness, irregularities, and variations in height, shape, or texture of a surface at a micro or macro scale. In the present disclosure, the rough surface morphology of the Pb particles includes, but is not limited to, bumps, ridges, valleys, peaks, or irregular shapes that may be randomly distributed or organized in a specific pattern. Additionally, the surface roughness may be determined by roughness average (Ra), root mean square (RMS) roughness, or peak-to-valley height. Roughness average (Ra) is calculated by averaging the surface roughness of at least 5, and preferably at least 10, representative locations spaced approximately evenly across the portion of the article carrying the Pb particles. In some embodiments, it is preferred to measure the thickness at representative points across the longest dimension of the portion of the article that is covered with the Pb particles. The standard deviation of roughness is found by calculating the standard deviation of the local average roughness across at least 5, and preferably at least 10, representative locations spaced approximately evenly across the portion of the article carrying the Pb particles. Arithmetic average roughness (Sa) is the areal (3D) equivalent of two-dimensional Ra. Sa generally refers to the average height of all measured points in the areal measurement.
In some embodiments, the Pb layer has an arithmetic average roughness (Sa) in a range of 2.5 to 3.8 μm, preferably 2.7 to 3.6 μm, preferably 2.9 to 3.4 μm, or even more preferably 3.1 to 3.2 μm. Other ranges are also possible.
In some embodiments, the Pb layer has a root mean square roughness (Sq) in a range of 3.0 to 5.7 μm, preferably 3.2 to 5.5 μm, preferably 3.4 to 5.3 μm, preferably 3.6 to 5.1 μm, preferably 3.8 to 4.9 μm, preferably 4.0 to 4.7 μm, preferably 4.2 to 4.5 μm, or even more preferably 4.3 to 4.4 μm. Other ranges are also possible.
In some embodiments, the electrode has a current density of 40 to 100 mA/cm2, more preferably 75 to 85 mA/cm2, and yet more preferably 80 mA/cm2 at a potential of −0.5 to −0.7 Volt (VAg/AgCl). As used herein, the term “current density” generally refers to the amount of current travelling per unit cross-section area. One of the important factors that affect the kinetics is the Tafel slope. The Tafel slope shows how efficiently an electrode can produce current in response to a change in applied potential. Therefore, a low Tafel slope suggests that less overpotential is required to get a high current. In some embodiments, the electrode has a Tafel slope of 200 to 360 millivolts per decade (mV/decade), more preferably 216 to 345 mV/decade, and yet more preferably 343 mV/decade for CO production in a salt solution at a scan rate of 5 to 50 millivolts per second (mV/s), more preferably 8 to 35 mV/s, and yet more preferably 10 to 30 mV/s.
The crystalline structures of the pristine Pb, and the ORC-treated Pb electrode at 5, 10, 20, and 50 cycles in 0.3 M KCl electrolyte solution may be characterized by X-ray diffraction (XRD), respectively. In some embodiments, the XRD patterns are collected in a Bruker-D2 Phaser bench-top x-ray diffractometer equipped with a Cu-Kα radiation source (2=0.15416 nm) for a 20 range extending between 5 and 90°, preferably 15 and 80°, further preferably 30 and 60° 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.
An X-ray diffraction (XRD) pattern of the pristine Pb, and the ORC-treated Pb electrode at 5, 10, 20, and 50 cycles in 0.3 M KCl electrolyte solution is illustrated in
In some embodiments, the ORC-treated Pb electrode has a first intense peak with a 2θ value in a range of 28 to 30°, preferably about 29° in the XRD spectrum as depicted in
The crystalline structures of the pristine Pb, and the ORC-treated Pb electrode at 5, 10, 20, and 50 cycles in 0.3 M KCl electrolyte solution may be characterized by energy-dispersive X-ray (EDX) spectroscopy, respectively. In some embodiments, the EDX spectrum is collected in a Quattro-S Ultra-versatile high-resolution field emission scanning electron microscope (FESEM) fitted with energy-dispersive x-ray spectrometer at an acceleration voltage of 5 to 30 kV, or preferably about 15 kV.
Referring to
At step 82, the method 80 includes disposing a lead sheet on the surface of the transparent substrate to form a Pb-coated substrate having the Pb layer. In some embodiments, lead powder, or lead alloy (major proportion being lead), may be disposed on the substrate instead of a lead sheet. In some embodiments, other elements selected from Ni, Al, Cu, Fe, Ag, Zn, Sn, Sb, Ti, In, V, Cr, Co, C, Ca, Mo, Au, P, W, Rh, Mn, B, Si Ge, Se, Ln, Ga, Ir, and an alloy or a mixture of two or more elements, may be disposed on the surface of the transparent substrate, in combination with the lead sheet. The lead sheet may be disposed on the substrate by any of the methods conventionally known in the art.
At step 84, the method 80 includes immersing the Pb-coated substrate into a solution including a potassium salt. In some embodiments, the potassium salt may include one or more of potassium hexafluorophosphate, potassium chloride, potassium fluoride, potassium sulfate, potassium carbonate, potassium phosphate, potassium nitrate, potassium pyrophosphate, potassium dodecylbenzenesulfonate, potassium dodecylsulfate, potassium metaborate, potassium borate, potassium molybdate, potassium tungstate, potassium bromide, potassium nitrite, potassium iodate, potassium iodide, potassium silicate, potassium oxalate, potassium aluminate, potassium methylsulfonate, potassium acetate, potassium dichromate, potassium hexafluoroarsenate, potassium tetrafluoroborate, potassium perchlorate, potassium trifluoromethanesulfonylimide, and potassium trifluoromethanesulfonate. In some further embodiments, the potassium salt may be potassium chloride (KCl). In a preferred embodiment, KCl may be substituted by AgCl, HCl, PbCl2. In some preferred embodiments, the KCl is present in the solution at a concentration of 0.001 to 1 molar (M), more preferably 0.01 to 0.9 M, preferably 0.1 to 0.8, preferably 0.2 to 0.7 M, preferably 0.3 to 0.6 M, or even more preferably 0.4 to 0.5 M.
Other ranges are also possible.
At step 86, the method 80 includes applying a cyclic voltammetry (CV) potential to the Pb—coated substrate. The CV is performed for 1 to 100 cycles, preferably 10 to 80 cycles, or even more preferably 20 to 50 cycles. The CV potential applied is in a range of from −0.5 to −0.7 V Ag/AgCl, preferably −0.55 to −0.65 VAg/AgCl, or even more preferably about −0.6 VAg/AgCl, at a scan rate of 10 to 30 mV/s, preferably 15 to 25 mV/s, or even more preferably about 20 mV/s. Other ranges are also possible. Applying the CV potential to the Pb-coated substrate forms substantially irregular octahedral-shaped Pb particles on the surface of the Pb layer. The irregular octahedral-shaped Pb particles have an average particle size of about 0.5 to 2 μm, or even more preferably about 1 μm. In some preferred embodiments, the irregular octahedral-shaped Pb particles are uniformly distributed on the surface of the Pb layer.
At step 88, the method 80 includes removing the Pb-coated substrate from the electrolyte solution, washing, and drying to form the electrode. The Pb-coated substrate may be washed, and further dried to a temperature of 60-90° C., preferably 65 to 85° C., preferably 70 to 80° C., or even more preferably about 75° C. for 1-3 hours, or even more preferably about 2 hours to form the electrode. The drying can be done 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.
At step 92, the method 90 includes charging an electrolyte to an electrochemical cell. The electrolyte includes an aqueous solution. The aqueous solution may include water and an inorganic base. The water may be tap water, distilled water, bi-distilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In one embodiment, the water is bi-distilled to eliminate trace metals. Preferably the water is bi-distilled, deionized, deionized distilled, or reverse osmosis water and at 25° C. The aqueous solution contains a salt having a concentration of 0.05 to 2 M, preferably 0.1 to 1.5 M, preferably 0.5 to 1 M, or even more preferably about 1 M. In a preferred embodiment, the salt includes potassium bicarbonate (KHCO3), and sodium bicarbonate (NaHCO3). In some embodiments, the salt may include ammonium bicarbonate, barium carbonate, calcium carbonate, magnesite, sodium percarbonates, sodium carbonate.
In some embodiments, the aqueous solution may include at least one base selected from the group consisting of an alkaline earth metal hydroxide and an alkali metal hydroxide. The base may be selected from the group consisting of an alkaline earth metal hydroxide such as beryllium hydroxide (Be(OH)2), magnesium hydroxide (Mg(OH)2), strontium hydroxide (Sr(OH)2), and calcium hydroxide (Ca(OH)2) and an alkali metal hydroxide such as lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH) and rubidium hydroxide (RbOH), and cesium hydroxide (CsOH).
The electrochemical cell is preferably a H-type sealed two-compartment electrolytic cell separated by a proton exchange membrane. In some embodiments, the proton exchange membrane may be a perfluorinated proton exchange membrane having a thickness of less than 0.01 inch, preferably less than 0.05 inch, preferably less than 0.01 inch, or even more preferably less than 0.007 inch. Each compartment of the electrochemical cell includes a working electrode, a counter electrode, and a reference electrode. The working electrode may be the electrode containing the Pb layer. The 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. 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. The counter electrode preferably should not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable contamination of either electrode. In some embodiments, each compartment of the electrochemical cell is in fluid communication via the proton exchange membrane.
In some embodiments, the working electrode and the counter-electrode may be connected to each other by way of electrical interconnects that allow for the passage of current between the electrodes, when a potential is applied between them. In a preferred embodiment, the electrocatalyst (which forms the working electrode) and the counter electrode may be at least partially submerged in the water and are not in physical contact with each other. In an embodiment, the working electrode and the counter-electrode may have the same or different dimensions. The working electrode and the counter-electrode may be arranged as obvious to a person of ordinary skill in the art.
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 CO2—containing gas composition. The CO2—containing gas composition may include other gases known in the art. CO2 is present in the CO2—containing gas composition at a concentration of at least 90 wt. % based on the total weight of the CO2—containing gas composition. The CO2—containing gas composition is introduced into the electrochemical cell at a rate of 1 to 50 standard cubic centimeters per minute (sccm), more preferably 20 to 40 sccm, and yet more preferably 30 sccm. Other ranges are also possible.
At step 96, the method 90 includes during the passing, simultaneously applying a potential between the working electrode and the counter electrode in the electrochemical cell via the electrolyte to form a product including carbon monoxide (CO) and a formate species. The formate species includes at least one of sodium formate, potassium formate, and formic acid. The product further includes hydrogen, ethanol, and methanol. The formate species is bonded to the surface of the electrode. In some embodiments, the formate species is bonded to the Pb particles of the electrode via at least one interaction selected from the group consisting of Van der Waals forces, London dispersion forces, dipole-dipole interactions, hydrogen bonding, ion-dipole interactions, ion-ion interactions, solvent-solute interactions, and ionic bonding.
The performance of the Pb-electrodes of the present disclosure towards formate production during the electrochemical CO2 reduction reaction (CO2RR) was evaluated; the results indicate that compared to pristine Pb which achieved a maximum current density of ˜20 milliamperes per square centimeter (mA/cm2) and an FE of formate ˜40%, the same is demonstrated by carrying out multiple oxidation-reduction cycling (ORC) on the Pb electrodes, it is possible to tune the selectivity. The ORC-treatment of the Pb electrodes results in the formation of an array of irregular octahedral-like Pb particles with a significantly enhanced electrocatalytic performance, reaching a maximum current density of ˜80 mA/cm2. The ORC-treated electrode demonstrates very high selectivity to CO reaching a faradic efficiency (FE) of 89%, and very high stability within 24 hrs of continuous electrolysis with an average CO production rate of 30 micromoles per hour per square centimeter (μ.mol hr−1 cm−2).
The method 90 has a CO faradaic efficiency (FE) of about 89% based on the total charge passed during the applying the potential. As used herein, the term ‘faradaic efficiency (FE)’ refers to the overall selectivity of an electrochemical process and is defined as the amount (moles) of collected product relative to the amount that can be produced from the total charge passed, expressed as a fraction or a percent. In some embodiments, the method 90 has a CO production rate of from 25 to 35 μmol hr−1 cm−2, more preferably 30 to 32 μmol hr−1 cm−2, 30.5 μmol hr−1 cm−2 at an overpotential of −0.8 to −0.6 VRHE.
The electrocatalyst of the present disclosure may also be used in the field of batteries, fuel cells, photochemical cells, water splitting cells, electronics, water purification, hydrogen sensors, semiconductors (such as field-effect transistors), magnetic semiconductors, capacitors, data storage devices, biosensors (such as redox protein sensors), photovoltaics, liquid crystal screens, plasma screens, touch screens, OLEDs, antistatic deposits, optical coatings, reflective coverings, anti-reflection coatings, and/or reaction catalysis.
The following examples describe and demonstrate exemplary embodiments of the electrode described herein. The examples are provided solely for the purpose of 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.
All reagents and chemicals used were of high purity (>99%) and used as received without further purifications. These chemicals include potassium chloride, KCl, potassium bicarbonate, KHCO3 (Fluka chemicals), sodium dihydrogen phosphate, NaH2PO4, sodium hydrogen phosphate, Na2HPO4 (Fisher Scientific™), lead metal sheet 1/16″ (technical grade, Fisher Scientific™), absolute ethanol (Merck) and 5 wt. % Nafion D520 (Fuel cell store). Ultra-pure deionized water (DI) was used throughout the experiments.
Commercial Pb sheets were first cleaned with absolute ethanol followed by de-ionized water and then electrochemically roughened by carrying out multiple cycles (5, 10, 20, 50) of cyclic voltammetry (CV) in 0.3 M KCl electrolyte. The CV scans were conducted in the potential range of −0.5 to −0.7 V (vs. Ag/AgCl) at a scan rate of 20 mV/s. The roughened Pb sheets were rinsed several times with DI water and stored in a desiccator. The morphology of the electrodes was examined using a Quattro-S Ultra-versatile high-resolution field emission scanning electron microscope (FESEM) fitted with energy-dispersive X-ray spectrometer at an acceleration voltage of 15 kilovolts (kV) (manufactured by Thermo Scientific™ Waltham, Massachusetts), whereas the X-ray diffraction patterns were recorded on D2 Phaser bench-top X-ray diffractometer (manufactured by Bruker, Billerica, Massachusetts, United States) using CuKα radiation in the 20 range of 10°-70°.
The electrodes were characterized by potentiostat electrochemical impedance spectroscopy (EIS) measurements in 5 millimolar (mM) of the redox-active potassium ferricyanide, K3 [Fe(CN) 6] containing 1.0 M KCl as supporting electrolyte. Impedance spectra were measured in the range of frequency 100 kilohertz (kHz) to 100 millihertz (mHz) at an amplitude of 10 mV root mean square (RMS). Electrocatalytic reduction experiments were conducted by controlled potential coulometry in a sealed two-compartment H-type electrochemical cell separated by a Nafion-117 proton exchange membrane. The electrode assembly includes the lead sheet (1 centimeter (cm) x1 cm×0.2 millimeters (mm)) as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. 0.5 M KHCO3 served as the anode and cathode electrolyte, respectively. Before the electrolysis, high-purity CO2 (99.999%) was passed into the electrolyte for 30 min at a flow rate of 30 standard cubic centimeters per minute (sccm), then maintained at 5 sccm during the electrolysis to remove residual air and ensure continuous CO2 supply. The gas products were analyzed online on Clarus 580 gas chromatograph (GC) (manufactured by Perkin Elmer, Waltham, Massachusetts, United States) equipped with Hayesep and molecular sieve columns and thermal conductivity (TCD) and flame ionization (FID) detectors, respectively.
Headspace products were injected into the GC periodically at 20 min intervals by an automated sampler. The electrolyte solution was collected at the end of 1 h electrolysis and analyzed for liquid products. Alcohols were analyzed on 7980A GC (manufactured by Agilent, 5301 Stevens Creek Blvd) equipped with supelcowax 10 (60 m×250 μm×0.25 μm) column and a flame ionization detector (FID) detector. Products were carried through the column at a flow rate of 1.2 milliliters per minute (mL/min). Formate was detected on a Compact IC flex 930 ion-exchange chromatography (Metrohm instruments) equipped with a Metrosep A 5-250 column (length 250 mm, ID 4 mm) and Metrosep supp. ⅘ pre-column (length 5 mm, ID 4 mm). 3.2 mmol/L Na2CO3 and 1.0 mmol/L NaHCO3 were used as eluents and 100 millimole per liter (mmol/L) H2SO4 as suppressant at a flow rate of 0.70 milliliter per minute (mL/min). 20 μL of the sample was injected at each analysis time.
The faradaic efficiency (FE) of the products was estimated using equation (1) [B. Jiang, X.-G. Zhang, K. Jiang, D.-Y. Wu, and W.-B. Cai, “Boosting Formate Production in Electrocatalytic CO2 Reduction over Wide Potential Window on Pd Surfaces,” J. Am. Chem. Soc., vol. 140, no. 8, pp. 2880-2889 Feb. 2018, which is incorporated herein by reference in its entirey]:
Where, Z represents the number of transferred electrons during the electrolysis, c is the molar concentration of the products (mol/L) as detected on GC or IC, V is the total volume of the gas or the liquid products (L), F is the Faraday constant (96485 coulombs per mole (C mol-1) of electrons), and Q is the overall charge over the period of 1 h electrolysis.
All the potentials reported in the present disclosure were converted to the reversible hydrogen electrode, RHE using equation (2).
Where, EAg/AGClO was taken as 0.228 V, and the pH of CO2—saturated 0.5 M KHCO3 was measured as 7.47.
DFT calculations were conducted using the Pople's 6-311G* basis set for the non-metallic atoms and the Stuttgart/Dresden (SDD) basis set for Pb atoms on Gaussian 09. The Grimme's dispersion correction (DFT-D3) was employed to account for the long-range Van der Waals interactions [S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, “A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu,” J. Chem. Phys., vol. 132, no. 15, p. 154104, 2010; and S. Grimme, S. Ehrlich, and L. Goerigk, “Effect of the damping function in dispersion corrected density functional theory,” J. Comput. Chem., vol. 32, no. 7, pp. 1456-1465, 2011, each of which is incorporated herein by reference in its entirety]. A simple unit cell of PbO, including 4 atoms of Pb and 4 atoms of O, was chosen to represent a catalyst surface. Full geometry optimizations were carried out to the minimum without imposing symmetry constraints on the potential energy surface [P. Kumar et al., “Alpha lead oxide (α-PbO): a new 2D material with visible light sensitivity,” Small, vol. 14, no. 12, p. 1703346, 2018, which is incorporated herein by reference in its entirety]. The self-consistent field convergence criterion was set to 1×10−8 Hartree during the optimizations, whereas the average root means square force on all atoms thresholds was set at 3.0×10−4 Hartree/Bohr. The adsorbed intermediates, HCOO* and *COOH pathways, were considered, and the free energy changes at each step were computed.
Adsorption of CO2 intermediates and products (Eads) was calculated using equation (4)
Where ET represents the free energy of the catalyst and the adsorbed reactants, intermediates, or products, and Epbo and Emol represents the free energies of the isolated PbO catalyst and the isolated reactants, intermediates, and products, respectively.
Furthermore, surface morphology and roughness parameters of the pristine Pb and ORC—treated Pb electrodes using a 3D optical profilometer by acquiring 3D optical profiles in a random location on each type of electrocatalyst (
The X-ray diffraction patterns of the pristine and the ORC-treated Pb electrodes are presented in
CV plots corresponding to 50 ORC cycles on the Pb electrode are presented in
Electrochemical reduction performance of the pristine Pb and ORC-treated electrodes was performed at different potentials to evaluate the FE values of CO2RR.
To gain in-depth kinetics of the tunning and enhanced catalytic activity of CO2 reduction to CO on ORC-treated electrodes from the electrochemical results, Tafel plots (over potential vs. log of the partial current density) were studied and are shown in
The long-term stability of ORC-treated electrodes catalyst for 24 h continuous and stable production of CO was achieved (
To further investigate the electrochemical CO2RR tunning ability of ORC-treated electrodes, first principle DFT simulations were performed to evaluate the free energy changes (AG (eV)) for ethanol, methanol, formate, and CO production on ORC-treated electrodes sites (
The present disclosure demonstrates an electrochemical surface roughening for efficient electrocatalytic CO2-to-CO conversion. These findings indicate morphology of the Pb electrode plays a critical role in CO selectivity. The results showed that the ORC-treated electrodes exhibit higher electrocatalytic activity and Faraday efficiency than the pristine Pb. The 50 cycles treated electrodes show the highest FECO up to 88% at −0.8 V vs. RHE and a production rate of 30.5 μmol h−1.cm−2. The superior electrocatalytic activity of ORC-treated electrodes can be attributed to the generated array of irregular octahedral-like Pb particles during the electrochemical surface roughening process, as shown in the SEM and 3D optical profiler analysis. These findings demonstrate morphology of the Pb electrode as a key parameter towards CO selectivity. Evaluation of density functional theory (DFT) calculation demonstrates *COOH intermediate as more likely to be generated and adsorb on ORC treated electrodes, verifying the charge acquisition of Pb2+ layer to enhances the activity and selectivity of CO product. The highly catalytic activity and selectivity of CO can be credited to the reduced formation energy barrier and enhanced adsorption behavior of *COOH intermediate.
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.
