CATALYST AND METHOD FOR ELECTROREDUCTION OF CARBON DIOXIDE, CARBON MONOXIDE, OR A COMBINATION THEREOF
20220372643 · 2022-11-24
Assignee
Inventors
Cpc classification
C25B11/053
CHEMISTRY; METALLURGY
Y02P20/133
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C25B11/055
CHEMISTRY; METALLURGY
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
B05D1/60
PERFORMING OPERATIONS; TRANSPORTING
International classification
B05D1/00
PERFORMING OPERATIONS; TRANSPORTING
C25B11/055
CHEMISTRY; METALLURGY
Abstract
There is provided a catalytic system including a fibrous hydrophobic substrate, a first layer having a first layer thickness including copper or copper alloy nanoparticles covering the polymeric substrate, and a second layer having a second layer thickness over the first layer and including amorphous nitrogen-doped carbon, wherein the catalytic system includes confined interlayer spaces defined by regions where the first layer and the second layer are spaced apart from each other. The catalytic system can be used for catalyzing the electrochemical reduction of carbon dioxide, carbon monoxide, or a combination thereof. Thus, there is also provided a method for the electrochemical reduction of carbon dioxide, carbon monoxide, or a combination thereof, using the catalytic system.
Claims
1. A catalytic system comprising a fibrous hydrophobic substrate, a first layer having a first layer thickness comprising copper or copper alloy nanoparticles covering the polymeric substrate, and a second layer having a second layer thickness over the first layer and comprising amorphous nitrogen-doped carbon, wherein the catalytic system comprises confined interlayer spaces defined by regions where the first layer and the second layer are spaced apart from each other.
2. The catalytic system of claim 1, wherein the fibrous hydrophobic substrate comprises polymeric nanofibers, carbon nanofibers, or a combination thereof.
3. The catalytic system of claim 1, wherein the fibrous hydrophobic substrate comprises at least one fluoropolymer.
4. The catalytic system of claim 1, wherein the fibrous hydrophobic substrate comprises polytetrafluoroethylene (PTFE).
5. The catalytic system of claim 1, wherein the fibrous hydrophobic substrate comprises a nanofibers membrane.
6. The catalytic system of claim 1, wherein the fibrous hydrophobic substrate comprises a nanofibers membrane having a pore size ranging from about 200 nm to about 700 nm.
7. The catalytic system of claim 1, wherein the fibrous hydrophobic substrate comprises nanofibers having a diameter ranging from about 50 nm to about 200 nm.
8. The catalytic system of claim 1, wherein the first layer thickness is from about 100 nm to about 500 nm and the second layer thickness is from about 20 nm to about 100 nm.
9. The catalytic system of claim 1, wherein the copper or copper alloy nanoparticles have a diameter ranging from about 20 nm to about 100 nm.
10. The catalytic system of claim 1, wherein in the second layer, the amorphous nitrogen-doped carbon comprises electron-donating nitrogen atoms.
11. The catalytic system of claim 1, wherein an atomic percentage of nitrogen in the second layer is from about 3% to about 50%.
12. The catalytic system of claim 1, wherein the second layer comprises pyridinic-N, pyrrolic-N and graphitic-N.
13. The catalytic system of claim 11, wherein a content of pyridinic-N is higher than a content of pyrrolic-N or graphitic-N.
14. The catalytic system of claim 1, wherein the second layer comprises pyridinic-N in an atomic percentage from about 10% to about 21%.
15. The catalytic system of claim 1, wherein the second layer comprises a plurality of pores extending through the second layer thickness.
16. The catalytic system of claim 15, wherein the pores in the second layer have an average diameter from about 5 nm to about 20 nm.
17. The catalytic system of claim 1, wherein the first layer and the second layer are spaced apart from each other in the confined interlayer spaces by a distance that is about 1 nm or below.
18. A membrane electrode assembly system comprising a cathode side and an anode side, wherein the cathode side comprises the catalytic system as defined in claim 1.
19. A method for electrochemical reduction of carbon dioxide, carbon monoxide, or a combination thereof, comprising: contacting a reactant gas comprising carbon dioxide, carbon monoxide, or a combination thereof, in the presence of an electrolyte, with a cathode comprising the catalytic system as defined in claim 1; applying a voltage to provide a current density to cause the carbon dioxide, carbon monoxide, or the combination thereof, in the reactant gas contacting the cathode, to be electrochemically reduced.
20. A method for electrochemical production of ethanol from carbon dioxide, carbon monoxide, or a combination thereof, comprising: contacting a reactant gas comprising carbon dioxide, carbon monoxide, or a combination thereof, in the presence of an electrolyte, with a cathode comprising the catalytic system as defined in claim 1; applying a voltage to provide a current density to cause the carbon dioxide, carbon monoxide, or the combination thereof in the reactant gas contacting the cathode, to be electrochemically converted into ethanol.
21. A process for producing a catalytic system as defined in claim 1, comprising: sputtering copper or the copper alloy onto the fibrous hydrophobic substrate to form the first layer; and sputtering the nitrogen-doped carbon onto the first layer to form the second layer.
Description
DESCRIPTION OF DRAWINGS
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DETAILED DESCRIPTION
[0120] The present description relates to a catalytic system for the electroreduction of carbon dioxide, carbon monoxide, or a combination thereof. The present description also relates to a method for electrochemically reducing carbon dioxide, carbon monoxide, or a combination thereof involving such catalytic system.
[0121] In some implementations, the catalytic system includes a fibrous hydrophobic substrate, a first layer having a first layer thickness including copper or copper alloy nanoparticles covering the polymeric substrate, and a second layer having a second layer thickness over the first layer and including amorphous nitrogen-doped carbon. The catalytic system further includes confined interlayer spaces defined by regions where the first layer and the second layer are space apart from each other.
[0122] As used herein the expression “fibrous hydrophobic substrate” refers to a substrate made of fibres onto which the copper-based catalyst can be applied. The fibrous substrate is hydrophobic meaning that the fibres, at least on their surface, can repel water. Hence, the fibres can include a hydrophobic coating to provide the hydrophobic properties to the fibrous substrate. In some implementations, the whole fibres can be made of a hydrophobic material.
[0123] Various material possessing hydrophobic properties can be employed and are known in the field. In some implementations, the fibrous hydrophobic material can include a hydrophobic organic polymer. In other implementations, the fibrous hydrophobic material can include carbon fibers. In some implementations, the fibrous hydrophobic material can include a combination of a hydrophobic organic polymer and carbon fibers, or any other hydrophobic material.
[0124] In some implementations, the fibrous hydrophobic substrate of the catalytic system can include one or more fluoropolymers as the hydrophobic material making of the fibers. An example of such fluoropolymer is polytetrafluoroethylene (PTFE).
[0125] In some implementations, the fibers of the hydrophobic substrate are in the nanometric size range. In other words, the hydrophobic substrate to which the copper-based layer and then the nitrogen-doped carbon layer are deposited, can include nanofibers made of a hydrophobic material. In some implementations, the fibrous hydrophobic substrate can thus include nanofibers made of the above-described hydrophobic materials, such as, fluoropolymer nanofibers (e.g., PTFE nanofibers), carbon nanofibers, or a combination thereof. In some implementations, the nanofibers can have a diameter ranging from about 50 nm to about 200 nm. Hence, in some implementations, the nanofibers can have a diameter of about 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, or any value between these values. In some implementations, the nanofibers can have an average diameter of about 100 nm. In some implementations, there is no limitation to the length of the nanofibers. The term “about” as used herein, indicates that a value includes the standard deviation of error for the device or method being employed in order to determine the value. In general, the terminology “about” is meant to designate a possible variation of up to 10%. Therefore, a variation of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% of a value is included in the term “about”. Unless indicated otherwise, use of the term “about” before a range applies to both ends of the range.
[0126] In some implementations, the fibrous hydrophobic substrate can be in the form of a membrane including the nanofibers described above. In other words, the substrate can include a plurality of nanofibers assembled to form a matrix presenting a porous structure. The presence of pores in the membrane can allow gas and/or electrolyte diffusion when the catalytic system is integrated into a membrane electrode assembly as will be described below. In some implementations, the nanofibers membrane that can be used as the hydrophobic substrate, can present a porous structure with a pore size ranging from about 200 nm to about 700 nm. Hence, in some implementations, the size of the pores in the porous hydrophobic nanofibers membrane can be about 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, or any value between these values. In some implementations, the pores of the nanofibers membrane can have an average pore size ranging from about 400 nm to about 500 nm. In some implementations, a nanofibers membrane having an average pore size of about 450 nm can be used as the substrate for the catalytic system.
[0127] As previously mentioned, the fibrous hydrophobic substrate (e.g., the nanofibers and/or nanofibers membrane described herein) is covered with a first conductive layer and then with a second layer over the first layer. By “covered”, one means that at least a portion of the fibrous hydrophobic substrate is coated with the first layer and the second layer. In some implementations, the whole surface of the fibrous hydrophobic substrate can be substantially coated with the first layer and then the second layer. In some implementations, minimal portions of the fibrous hydrophobic substrate remain uncoated.
[0128] The first layer covering the fibrous hydrophobic substrate can include a conductive metal such as copper. In some implementations, the first layer can include nanoparticles of a copper-based material, such as copper nanoparticles or copper alloy nanoparticles. When the first layer includes copper alloy nanoparticles, the alloy can include CuZn, CuAg, CuAu, to name a few examples. In some implementations, the metallic nanoparticles forming the first layer can have a diameter ranging from about 20 nm to about 100 nm. Hence, in some implementations, the diameter of the first layer nanoparticles can be about 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any value between these values. Even if the present description generally discusses a first layer including copper-based nanoparticles, it is worth noting that in some implementations, the first layer could include alternative conductive nanoparticles, as soon as the first layer and the nitrogen-doped carbon second layer disposed on the substrate can form confined interlayer spaces or regions, as will be explained in further detail below.
[0129] In some implementations, the first layer of the catalytic system, namely the layer coated over the fibrous hydrophobic substrate can be characterized by a thickness ranging from about 100 nm to about 500 nm. In some implementations, the thickness of the first layer can range from about 100 nm to about 400 nm, or from about 100 nm to about 300 nm, or from about 100 nm to about 250 nm, or from about 150 nm to about 250 nm. In some implementations, the thickness of the first layer can be about 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or any value between these values. Although the thickness of the first layer can vary, an average thickness having a value mentioned herein can provide a suitable catalytic system. One will also understand that the thickness of the first layer can slightly vary in different regions of the substrate's surface, with regions having slightly higher or smaller thickness than other regions.
[0130] The second layer that is present over the first layer on the fibrous hydrophobic substrate (e.g., the nanofibers and/or nanofibers membrane described herein) includes nitrogen-doped carbon and more particularly amorphous nitrogen-doped carbon. In some implementations, the second layer includes amorphous nitrogen-doped carbon including electron-donating nitrogen atoms. By “nitrogen-doped”, it is meant that nitrogen atoms are introduced in the solid structure of the basis carbon material. For instance, some carbon atoms can be replaced with nitrogen atoms in the material structure. In some implementations, the atomic percentage of nitrogen in the second layer can be from about 3% to about 50%. For instance, the atomic percentage of nitrogen in the second layer can be from about 10% to about 50%, or from about 10% to about 50%, or from about 20% to about 50%, or from about 20% to about 45%, or from about 25% to about 45%, or from about 25% to about 40%. In some implementations, the atomic percentage of nitrogen in the second layer can be from about 26% to about 39%. In some implementations, the atomic percentage of nitrogen in the second layer can be 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, or 39%. In some implementations, the nitrogen atoms in the nitrogen-doped carbon material can be included in pyridinic, pyrrolic and/or graphitic structures. In other words, the second layer can include pyridinic-N, pyrrolic-N, graphitic-N, or any combination thereof. In some implementations, one can find nitrogen atoms being pyridinic-N, pyrrolic-N and graphitic-N in the second layer. In certain implementations, the content of pyridinic-N, in the second layer, is higher than the content of pyrrolic-N or graphitic-N. In further implementations, the second layer can include pyridinic-N in an atomic percentage from about 10% to about 21%. Hence, in some implementations, the pyridinic-N can be present in an atomic percentage of about 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or 21%, in the second layer. The presence of pyridinic-N can enhance the electron-donating ability of the nitrogen-doped carbon layer due to the presence of the lone electron pair of the nitrogen atom in the plane of the carbon matrix. This can further allow increasing at least ethanol Faradaic efficiency (FE) during electroreduction of carbon dioxide and/or carbon monoxide, particularly carbon dioxide, using the present catalytic system.
[0131] As mentioned above, the nitrogen-doped carbon material in the second layer includes amorphous nitrogen-doped carbon. By “amorphous”, it is meant that the nitrogen-doped carbon mainly includes non-crystalline nitrogen-doped carbon. The second layer thus mostly includes nitrogen-doped carbon presenting an amorphous structure, but the presence of crystalline structure is not excluded. In some implementations, the second layer can only include amorphous nitrogen-doped carbon. The second layer including nitrogen-doped carbon being amorphous, can be beneficial to the catalytic system. For instance, it was observed that CO.sub.2 reduction using a catalytic system as described herein, including nitrogen-doped carbon with an amorphous structure can improve ethanol selectivity vs. ethylene.
[0132] In some implementations, the second layer of the catalytic system can have a thickness that can be lower than the thickness of the first layer of copper-based nanoparticles onto which it is applied, although both layers could have substantially the same thickness. In some implementations, the second layer thickness can range from about 20 nm to about 100 nm. Hence, in some implementations, the thickness of the second layer can range from about 20 nm to about 90 nm, or from about 20 nm to about 80 nm, or from about 20 nm to about 70 nm, or from about 30 nm to about 70 nm, or from about 40 nm to about 60 nm. In some implementations, the thickness of the second layer can be about 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any value between these values. Although the thickness of the second layer can be varied, an average thickness having a value mentioned herein can provide a suitable catalytic system. One will also understand that the thickness of the second layer can slightly vary in different regions over the substrate, with regions having slightly higher or smaller thickness than other regions.
[0133] In some implementations, the second layer in the catalytic system can further include a plurality of pores extending through a thickness thereof. In other words, pores can be present in the second layer, which can extend from the surface of the second layer that is opposite to the surface disposed over the first layer, towards the first layer. These pores present in the second layer can have an average diameter ranging from about 5 nm to about 20 nm. In some implementations, the pores in the second layer can have an average diameter ranging from about 5 nm to about 15 nm. Hence, in some implementations, the average size of the pores in the second layer can be about 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, or 20 nm.
[0134] Another characteristic of the catalytic system described herein, is the presence of confined interlayer spaces between the first layer and the second layer. More particularly, the catalytic system includes regions where the first layer and the second layer are spaced apart from each other separated by regions where both layers are adjoined. The regions where the layers are spaced apart define the so-called confined interlayer spaces. In other words, the structure of the catalytic system is such that the second layer is at a certain distance from the first layer in certain regions of the bilayer assembly. The presence of confined regions can enable molecules and solutions to intercalate between the two layers, which, in turn, can allow to enhance the activity of the catalytic system. More specifically, in the case of CO.sub.2 and/or CO electroreduction, the CO.sub.2 and/or CO molecules, reaction intermediates (which can include CO molecules when the reactant gas includes CO.sub.2) and products can be confined within the interlayer spaces. Hence, the reaction can occur within the confined interlayer spaces and such spaces can thus function as nanoreactors. In some implementations, in the present catalytic system, the first layer and the second layer can be spaced apart from each other, in the confined interlayer spaces, by a distance that is about 1 nm or below. In some implementations, the distance between the first layer and the second layer in the confined interlayer spaces can be from about 0.6 nm to about 1 nm, or from about 0.6 nm to about 0.9 nm. Hence, in some implementations, the distance between the first layer and the second layer in the confined interlayer spaces can be about 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, or 1 nm.
[0135] A schematical view of the catalytic system according to some implementations is shown in
[0136] The catalytic system according to the present disclosure can be prepared using any techniques known in the field. In some implementations, the catalytic system can be produced by a sputtering method, such as magnetron sputtering deposition. Hence, in a first step, the copper or copper alloy is sputtered onto the fibrous hydrophobic substrate to form the first layer of copper or copper alloy nanoparticles. A subsequent step can include sputtering the nitrogen-doped carbon onto the first layer to form the second layer of the catalytic system. In some implementations, the first layer can be formed from a copper or copper alloy target, in an argon environment, at a sputtering rate that can vary from about 0.5 Å s.sup.−1 to about 1.5 Å s.sup.−1. In some implementations, the sputtering rate for forming the first layer can range from about 1 Å s.sup.−1 to about 1.5 Å s.sup.−1, or from about 1 Å s.sup.−1 to about 1.2 Å s.sup.−1. Hence, in some implementations, the copper or copper alloy sputtering rate for forming the first layer can be about 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5 Å s.sup.−1. In further implementations, the second layer can be formed from a carbon target (e.g., carbon graphite), in a N.sub.2 and argon environment, at a sputtering rate that can vary from about 0.01 Å s.sup.−1 to about 0.1 Å s.sup.−1. In some implementations, the sputtering rate for forming the second layer can range from about 0.01 Å s.sup.−1 to about 0.07 Å s.sup.−1, or from about 0.03 Å s.sup.−1 to about 0.07 Å s.sup.−1. Hence, in some implementations, the carbon target sputtering rate for forming the second layer can be about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 Å s.sup.−1. The flow rate of the N.sub.2 gas during sputtering of the carbon target can be adjusted to obtain a desired nitrogen atomic percentage in the second layer. In some implementations, carbon sputtering can be performed at a flow rate ratio of N.sub.2 to argon from about 2/20 sccm to about 20/20 sccm (sccm=standard cubic centimetres per minute). In some implementations, the flow rate ratio of N.sub.2 to argon during sputtering of the carbon target can thus be about 2/20, 3/20, 4/20, 5/20, 6/20, 7/20, 8/20, 9/20, 10/20, 11/20, 12/20, 13/20, 14/20, 15/20, 16/20, 17/20, 18/20, 19/20, or 20/20 sccm. In some implementations, the time during which sputtering is performed, either for forming the first layer and/or the second layer, can be adjusted for obtaining a proper layer thickness. For instance, it can be possible to increase the first and/or second layer thickness, at a fixed sputtering rate, by increasing the sputtering period. Hence, a variety of catalytic system can be produced using the present method, by varying at least the nature of the starting material for the first layer (i.e., pure copper or a copper alloy target), the sputtering rates, the sputtering time and/or flow rate of the N.sub.2 gas during sputtering of the carbon target.
[0137] The catalytic system according to the present disclosure can be included in a membrane electrode assembly (MEA) system that can be used to perform electroreduction of carbon dioxide, carbon monoxide, or a combination thereof. In some implementations, one can refer to a “MEA electrolyser”. In some implementations, the MEA system or MEA electrolyser can be used to electrochemically reduce carbon dioxide, carbon monoxide, or a combination thereof, to then produce at least one of ethanol, n-propanol, ethylene, acetate and/or acetic acid, formate and/or formic acid, methane, and hydrogen. In some implementations, CO can be produced from the electroreduction of carbon dioxide using the MEA system. In further implementations, MEA system can particularly be used for the electroreduction of carbon dioxide, carbon monoxide, or a combination thereof, into ethanol. In some implementations, the MEA system or MEA electrolyser can particularly be used to electrochemically reduce carbon dioxide to produce ethanol.
[0138] In some implementations, the MEA system is provided with means to supply a reactant gas including CO.sub.2, CO, or a combination thereof, to the cathode side of the MEA system. In some implementations, the reactant gas can be flowed through a humidifier before being supplied to the cathode compartment. The humidifier can for instance include deionized water. Hence, a humidified reactant gas can be supplied to MEA system, and then be contacted with the catalytic system of the present disclosure, to allow electroreduction of CO.sub.2 and/or CO.
[0139] According to some implementations, the present disclosure thus further relates to a method for the electrochemical reduction of CO.sub.2, CO, or a combination thereof, involving the use of the catalytic system or of the MEA system described herein. The method includes contacting a reactant gas including carbon dioxide, carbon monoxide, or a combination thereof, in the presence of an electrolyte, with a cathode including the catalytic system of the present disclosure and applying a voltage to provide a current density to cause the CO.sub.2 and/or CO in the reactant gas contacting the cathode, to be electrochemically reduced. As mentioned above, the gas can preferably be humidified before contacting with the cathode. The electrolyte that can be used for electroreduction of CO.sub.2 and/or CO can be an alkaline aqueous solution, preferably including a strong base. In some implementations, the electrolyte can include a KHCO.sub.3 or a KOH aqueous solution. The voltage applied for the electroreduction reaction can be determined and optimized to enhance selectivity towards a desired product
[0140] In some implementations, the method can particularly be used for the production of ethanol and can be performed by contacting the reactant gas including carbon dioxide, carbon monoxide, or a combination thereof, in the presence of an electrolyte, with a cathode including the catalytic system as defined in the present disclosure, followed by applying a voltage to provide a current density to cause the carbon dioxide, carbon monoxide, or the combination thereof in the reactant gas contacting the cathode, to be electrochemically converted into ethanol. The gas can be humidified before contacting with the cathode. The electrolyte can be an alkaline aqueous solution, preferably including a strong base. In some implementations, the electrolyte can include a KHCO.sub.3 or a KOH aqueous solution. The voltage for the electroreduction reaction can be determined and optimized to enhance selectivity towards ethanol. For example, the voltage that can be applied for electrochemically reducing CO.sub.2 into ethanol can be from about −3.0 to about −4.0 volts.
[0141] In some implementations, the reactant gas that is subjected to electroreduction can be a raw gas, an enriched gas, a diluted gas or a pure gas from many different CO.sub.2 and/or CO sources. In some implementations, the electrochemical reduction method using the catalytic system of the present disclosure, can be employed for reducing CO.sub.2 and/or CO present in an exhaust gas produced from industrial and/or agricultural processes. For instance, the reactant gas that can be treated in the electroreduction method disclosed herein, can include a gas resulting from the combustion of fossil fuels, a flue gas from stacks, an off-gas (i.e., a gas emitted as the by-product of a chemical process) or a biogenic CO.sub.2 gas resulting from the combustion, harvest, digestion, fermentation, decomposition or processing of biologically based materials other than fossil fuels. In some implementations, the reactant gas can be pure or enriched CO.sub.2 derived from any of the above-mentioned sources and can be produced from a raw CO.sub.2-containing gas by various methods such as absorption-desorption (e.g., amine scrubbing), adsorption-desorption (e.g., using adsorbents capable to catch and release CO.sub.2), among others. In some implementations, the CO.sub.2-containing gas can be captured from the atmosphere (air) using various technologies (e.g., direct air capture). In some implementations, the reactant gas, that can be derived from any of the above-mentioned sources, including from air, can be diluted before electroreduction. In some implementations, pure CO.sub.2 derived from any of the above-mentioned sources can be employed in the electroreduction process. In some implementations, a CO-containing gas can be used as the reactant gas. In some implementations, the CO-containing gas can be obtained from a CO.sub.2-containing gas. For instance, the reactant gas can include CO which results from the reduction of CO.sub.2. In some implementations, the reactant gas can include both CO.sub.2 and CO.
EXAMPLES & EXPERIMENTATION & FINDINGS
[0142] The following section relates to various experiments that were conducted in the course of this work.
1—Methods
[0143] Chemicals. Copper target (99.999%) and carbon graphite target (99.999%) were purchased from Kurt J. Lesker company (Certain commercial equipment, instruments, or materials are identified herein in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose). Potassium hydroxide (KOH) was purchased from Caledon Laboratory Chemical. Iridium (III) chloride hydrate (IrCl.sub.3-xH.sub.2O, 99.9%) and potassium bicarbonate (KHCO.sub.3, 99.5%) were purchased from Sigma-Aldrich. Anion exchange membrane (Fumasep FAB-PK-130), gas diffusion layer (GDL, Freudenberg H14C9), and titanium mesh were received from Fuel Cell Store. Sustainion® anion-exchange membrane (AEM) was received from Dioxide Materials; the membrane was activated in 1 M KOH aqueous solution for 24 hours and then washed with water before use. PTFE membrane with an average pore size of 450 nm was purchased from Beijing Zhongxingweiye Instrument Co., Ltd. Ni foam (1.6 mm thickness) was purchased from MTI Corporation. All chemicals were used as received. All aqueous solutions were prepared using deionized (DI) water with a resistivity of 18.2 MΩcm.sup.−1.
[0144] Electrodes. All the cathodes were prepared using a magnetron sputtering system. Typically, a Cu cathode was prepared by sputtering 200 nm Cu catalyst (Cu target, sputtering rate: ˜1.1 Å s.sup.−1) on a piece of PTFE membrane. The mass loading of Cu on PTFE membrane is 0.19 mg cm.sup.−2. By sputtering 50 nm of the N—C layer or the carbon layer (carbon graphite target, sputtering rate: ˜0.05 Å s.sup.−1) on sputtered Cu catalysts supported by PTFE, we can further obtain the N—C/Cu and C/Cu cathodes, respectively. N—C layers with different nitrogen contents were prepared by adjusting the flow rate ratio of N.sub.2 to Ar. For deposition of carbon, 26%, 34%, and 39% N—C layers on sputtered Cu catalysts, the flow rate ratios of N.sub.2 to Ar are set to be 0/20, 2/20, 6/20, and 20/20 (sccm=standard cubic centimetres per minute), respectively. Similarly, the 34% N—C gas diffusion electrode (GDE) was prepared by sputtering 50 nm of the N—C layer onto the GDL.
[0145] In flow cell, Ag/AgCl reference electrode (3 M KCl, BASi) and Ni foam were used as the reference electrode and anode, respectively. In MEA system, the iridium oxide supported on titanium mesh (IrOx/Ti mesh) used as the anode catalyst was prepared by a reported dip coating and thermal decomposition method (ref. 39).
[0146] Structural and compositional analyses. SEM images and the corresponding EDX elemental mapping were taken using Hitachi FE-SEM SU5000 microscope. HAADF-STEM images, and the corresponding EDX and electron energy loss spectroscopy (EELS) elemental mapping were taken using a Hitachi HF-3300 microscope at 300 kV and aberration-corrected FEI Titan 80-300 kV TEM/STEM microscope at 300 kV, with a probe convergence angle of 30 mrad and a large inner collection angle of 65 mrad to provide a nominal image solution of 0.7 Å. For STEM/TEM imaging, an ultrathin slice (˜100 nm) was prepared using the Leica UM7 ultramicrotome (Leica Microsystems Inc. in Buffalo Grove, Ill.). The slice was then transferred to a 100-mesh nickel grid for characterization. Cross-sectional SEM image and EDX elemental mapping was performed using Hitachi Dual-beam FIB-SEM NB5000. Structural characterization of cathodes was obtained using XRD (MiniFlex600) with Cu-Kα radiation. The surface compositions of cathodes were determined by XPS (model 5600, Perkin-Elmer) using a monochromatic aluminum X-ray source. In situ Raman measurements were operated using a Renishaw in Via Raman Microscope in a modified flow cell and a water immersion objective (63×) with a 785 nm laser. XAS measurement were conducted at 9BM beamline at Advanced Photon Source (APS, Argonne national laboratory, IL). Ex-situ XAS measurements were carried out at the BL731 beamline at the Advanced Light Source (ALS, Lawrence Berkeley National Laboratory, CA) and the SXRMB beamline at the Canadian Light Source. Athena and Artemis software included in a standard IFEFFIT package were used to process XAS data (ref. 40). WAXS measurements were carried out in transmission geometry at the CMS beamline of the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) office of the Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory. Samples were measured with an imaging detector at a distance of 0.177 m using X-ray wavelength of 0.729 Å. Nika software package was used to sector average the 2D WAXS images (ref. 41). Data plotting was done in Igor Pro (Wavemetrics, Inc., Lake Oswego, Oreg., USA).
[0147] Electrochemical measurements. In the flow cell, the electrochemical measurements were conducted in the three-electrode system at an electrochemical station (AUT50783). Prepared cathodes, anion exchange membrane, and nickel foam were positioned and clamped together via PTFE gaskets. Here nickel foam was used as the anode for oxygen evolution reaction (OER) in flow cell as nickel is a good OER catalyst in alkaline environment (ref. 42). 30 ml of electrolyte (1 M KOH aqueous solution) was introduced into the anode chamber between anode and membrane, as well as the cathode chamber between membrane and cathode, respectively. The electrolytes in cathode and anode were circulated by two pumps at the rate of 10 mL min.sup.−1. Meanwhile, CO.sub.2 gas (Linde, 99.99%) was continuously supplied to gas chamber located at the back side of cathode at the rate of 50 mL min.sup.−1. Through the pore of the cathode, gas could diffuse into the interface between cathode and electrolyte. The performance of cathodes was evaluated by performing constant-current electrolysis. All potentials were measured against an Ag/AgCl reference electrode (3 M KCl, BASi). In isotope-labelling experiment, the procedure was the same to the above experiment condition, except that .sup.13CO.sub.2 gas (Sigma-Aldrich, 99 atom % .sup.13C) was used as the supply gas.
[0148] Gas and liquid products were analyzed using gas chromatograph (PerkinElmer Clarus 600) equipped with thermal conductivity and flame ionization detectors, and NMR spectrometer (Agilent DD2 600 MHz) by taking dimethylsulfoxide (DMSO) as an internal standard, respectively. All the potentials were converted to values with reference to RHE using:
E.sub.RHE=E.sub.Ag/AgCl+0.210V+0.0591×pH
[0149] The ohmic loss between the working and reference electrodes was measured through electrochemical impedance spectroscopy (EIS) technique at the beginning of the electrolysis and 95% iR compensation was applied to correct the potentials manually.
[0150] Cu.sub.ECSA in catalysts was determined using Pd underpotential deposition in flow cell. N.sub.2-saturated solution of 100 mM HClO.sub.4+1 mM Pd(ClO.sub.4).sub.2 is used as the electrolyte. The cathode was held at −0.081 V.sub.RHE for 60 s and then cyclic voltammetry (CV) was recorded between −0.281 and 0.239 V.sub.RHE at 5 mV s.sup.−1. Pt foil was used as the anode. N.sub.2 (Linde, 99.998%) was continuously supplied to gas chamber of the flow cell. The electrolyte was not circulated during the CV measurement. The Cu.sub.ECSA in the catalyst is calculated from the charge associated with 2e.sup.− oxidation of monolayer of Pd adatoms coverage over Cu surface with a conversion factor of 310 μC cm.sup.−2 (ref. 43).
[0151] The MEA was a complete 5 cm.sup.2 CO.sub.2 electrolyzer (SKU: 68732), which was purchased from Dioxide Materials. The cathode catalyst (2.25 cm by 2.25 cm) was attached on the cathode side by the copper tape at the edge of the electrode and Kapton tapes was then attached on the top of copper tape to avoid its contact with the membrane/electrolyte. The activated Sustainion® membrane (3 cm×3 cm) and five pieces of IrO.sub.x/Ti mesh anode catalysts (2.25 cm by 2.25 cm) were put on the top of the cathode successively and then assembled together in an MEA as shown in
[0152] Calculation for energy conversion efficiency. In flow cell, ethanol energy conversion efficiency is calculated for the half-cell in cathode (EE.sub.cathodic half-cell). The overpotential of oxygen evolution is assumed to be 0. The ethanol EE.sub.cathodic half-cell can be calculated as follows (ref. 6):
where E.sub.applied is the potential used in the experiment, FE .sub.ethanol is the measured Faradaic efficiency of ethanol in percentage, and E.sub.ethanol=0.09 V.sub.RHE for CO.sub.2RR (ref. 44).
[0153] In MEA system, ethanol energy efficiency is calculated for full cell:
where E.sub.full-cell appied is the full-cell voltage applied in MEA system.
[0154] Theoretical methods. In this work, all the DFT calculations were carried out with a periodic slab model using the Vienna ab initio simulation program (VASP) (refs. 45-48). The generalized gradient approximation (GGA) was used with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional (ref. 49). The projector-augmented wave (PAW) method (refs. 50-51) was utilized to describe the electron-ion interactions, and the cut-off energy for the plane-wave basis set was 450 eV. In order to illustrate the long-range dispersion interactions between the adsorbates and catalysts, the D3 correction method by Grimme et al. was employed (ref. 52). Brillouin zone integration was accomplished using a 3×3×1 Monkhorst-Pack k-point mesh.
[0155] For the modelling of copper, the crystal structure was optimized, and the equilibrium lattice constants were found to be α.sub.Cu=3.631 Å. A 4-layer model was used with p(3×3) super cell with the 2 upper layers relaxed and 2 lower layers fixed. All the adsorption geometries were optimized using a force-based conjugate gradient algorithm, while transition states (TSs) were located with a constrained minimization technique (refs. 53-55). At all intermediate and transition states, one charged layer of water molecules was added to the surface to take the combined effects of field and solvation into account: six water molecules are added near the surface, with three facing toward the surface, and three parallel with the surface (ref. 56). To consider the effect from the confinement of carbon and nitrogen-doped carbon on Cu, one layer of graphene or nitrogen-doped graphene was added on Cu surface as shown in
[0156] A sampling framework based on the amorphous study by Deringer et al. (ref. 57) was further developed. 50 amorphous carbon and 100 amorphous N-doped carbon structures were investigated using the pipeline (
[0157] The pipeline was started with the structure of Cu surface with intermediate and water (Structure-1). We then added amorphous carbon structures on the surface of Structure-1 and allow DFT to fully relax the amorphous structures and water molecules. All the fifteen amorphous structures are considered from the amorphous study (in folder “DFT_relaxed_216 at”) by Deringer et al. (ref. 57). The formation energy per atom distribution of the optimized amorphous carbon are shown in
[0158] The top 10 most stable unreconstructed amorphous carbon structures were kept, and for each amorphous carbon structure, we randomly replaced 30% carbon with nitrogen ten times. This process generated 100 amorphous N-doped carbon structures, and the formation energy per atom distribution of the 100 structures are shown in
[0159] Local species concentration modelling. The system was modeled as a two-dimensional domain with 200 nm thickness of Cu and 50 nm thickness of porous N—C layer according to the catalyst, as well as electrolyte sub-domains (
2—DFT Calculations
[0160] The CO dimerization reaction (
[0161] To understand how the N—C layer affected CO dimerization, electron density difference plots were also generated for N—C/Cu and C/Cu with two adsorbed *CO in solution (
[0162] HOCCH* is the key intermediate that branches the ethylene pathway and the ethanol pathway (
3—Catalyst Synthesis and Characterization
[0163] N—C/Cu catalysts were fabricated via sputter deposition of a layer of Cu nanoparticles on the surface of polytetrafluoroethylene (PTFE) nanofibers (
[0164] Analysis of HAADF-STEM images of microtomed 34% N—C/Cu catalyst demonstrates that there are regions in which a gap is present between the Cu layer and the N—C layer; and there exist other regions in which these layers touch one another (
4—Investigation of CO.SUB.2 .Electroreduction
[0165] The N—C/Cu electrode was electrochemically tested (
[0166] Under a current density of 300 mA cm.sup.−2, the total C.sub.2+ FE on 34% N—C/Cu is up to 93% and an ethanol FE of (52±1) % is achieved with a conversion rate of (156±3) mA cm.sup.−2 at −0.68 V.sub.RHE after ohmic loss correction (
[0167] The electrochemically active area of Cu (Cu.sub.ECSA) in N—C/Cu and Cu catalysts was estimated using Pb underpotential deposition (Pb.sub.UPD) (
[0168] To explore the effects of the N—C layer on the performance of CO.sub.2RR, we also prepared 26% N—C/Cu, 39% N—C/Cu, and C/Cu on PTFE via similar sputtering methods (
[0169] To compare local pH at the Cu surface in N—C/Cu and Cu, we carried out the local species concentration modelling for cases with and without the N—C layer (
[0170] It is well known that the formation of multi-carbon products in CO.sub.2RR goes through the formation of the carbon monoxide (CO) intermediate, and then the further reduction of CO intermediates.sup.11,26,27. To gain insight into C—C coupling on 34% N—C/Cu and Cu electrodes during CO.sub.2RR, we acquired Raman spectra in situ and investigated the interactions between the catalytic surface and the *CO intermediate (
[0171] It was also found that, under the same potentials, the bands for the Cu—CO stretch exhibited a blueshift on the 34% N—C/Cu compared to Cu (
[0172] We also performed operando X-ray absorption spectroscopy (XAS) at the Cu K-edge to investigate the Cu chemical state during CO.sub.2RR. Under the current density of 300 mA cm.sup.−2, all copper oxides in different N—C/Cu and Cu catalysts are reduced to Cu(0) within the first 16 s, and then the valence state of Cu is maintained at zero throughout CO.sub.2RR (
[0173] It was noted that the ethanol FE shows a volcano-shape relationship with the increase of nitrogen contents under the same current densities (
[0174] Additionally, were also acquired ex-situ X-ray absorption near edge structure (XANES) spectra at the nitrogen K-edge on different N—C/Cu catalysts in total electron yield (TEY) mode, which provided information on the near-surface chemical states.sup.37 (
[0175] We integrated the 34% N—C/Cu catalyst into a membrane electrode assembly (MEA) system to evaluate its stability (
5—Supplementary Information
[0176] This section contains additional information on the experimental results reported above, including the tables mentioned in the discussion. In addition, further comments are provided with respect to
TABLE-US-00002 TABLE 2 Activation energies (E.sub.a) and enthalpy changes (ΔH) of CO dimerization on Cu, C/Cu, and N—C/Cu. E.sub.a (eV) ΔH (eV) Cu 0.72 0.65 C/Cu 0.77 0.62 N—C/Cu 0.70 0.52
TABLE-US-00003 TABLE 3 Reaction energies of the intermediate state (HOCCH*) to CCH* (ethylene pathway) and HOCHCH* (ethanol pathway) on Cu, C/Cu, and N—C/Cu, respectively. Ethylene pathway (eV) Ethanol pathway (eV) Cu −0.09 −0.11 C/Cu −0.09 −0.20 N—C/Cu 0.07 −0.03
[0177] The confinement effect of N—C layer on Cu(100) was considered by calculating the reaction energies of the ethylene and ethanol pathways on Cu(100) and N—C/Cu(100), as shown in Table 4. The results show that, compared to Cu(100), N—C/Cu(100) also tends to improve the ethanol selectivity vs. ethylene.
TABLE-US-00004 TABLE 4 Reaction energies of the ethylene and ethanol pathways and energy differences of the ethanol and ethylene pathways on Cu(100) and N—C/Cu(100), respectively. All the energies are in eV. Ethylene Ethanol Ethanol pathway − pathway pathway Ethylene pathway (eV) (eV) (eV) Cu(100) −0.26 −0.48 −0.22 N—C/Cu(100) −0.47 −0.76 −0.29
[0178] To understand the effects of different layers of graphite in CO.sub.2RR, CO dimerization, HOCCH* to CCH* (ethylene pathway), and HOCCH* to HOCHCH* (ethanol pathway) on C/Cu was investigated with different layers of graphite carbon (Table 5 and
TABLE-US-00005 TABLE 5 Reaction energies of ethylene and ethanol pathways and activation energies (E.sub.a) and enthalpy changes (ΔH) of CO dimerization on Cu with different layers of graphite. All the energies are in eV. Ethylene Ethanol CO dimerization Layer pathway pathway E.sub.a ΔH 1 −0.09 −0.20 0.77 0.62 2 −0.10 −0.21 0.77 0.63 3 −0.10 −0.21 0.77 0.63
[0179] To consider the effect of the double layer under negative potentials, we implemented the grand canonical quantum mechanics (GCQM) method (ref. 77) in JDFTX (ref. 78). The settings in the paper of Goddard and co-workers (ref. 79) were adapted. We set the ions to be 1.0 M KOH and the applied potential to be 0 V vs. standard hydrogen electrode (SHE). A comparison between the charged water model and the GCQM method is provided in Table 6. There are slight differences in absolute values, but the trends are in agreement: N—C/Cu is the best for CO dimerization, and both C/Cu and N—C/Cu favor ethanol. The high barrier of CO dimerization on C/Cu limits selectivity towards C.sub.2 products, agreeing with the presently reported experiment results.
TABLE-US-00006 TABLE 6 Energy differences of the ethanol and ethylene pathways (E.sub.ethanol pathway − E.sub.ethylene pathway), activation energies (E.sub.a (OC—CO)), and enthalpy changes (ΔH (OC—CO)) of CO dimerization using the charged water model and 1.0M KOH using GCQM. All the energies are in eV. Methods Energy (eV) Cu C/Cu N—C/Cu Charged water model E.sub.ethanol pathway − −0.02 −0.12 −0.11 E.sub.ethylene pathway E.sub.a (OC—CO) 0.72 0.77 0.70 ΔH (OC—CO) 0.65 0.62 0.52 1.0M KOH using GCQM E.sub.ethanol pathway − 0.09 −0.08 −0.01 E.sub.ethylene pathway E.sub.a (OC—CO) 0.82 0.85 0.77 ΔH (OC—CO) 0.78 0.73 0.61
[0180] In the main calculations, the distance between N—C layer and Cu layer (d.sub.N—C/Cu) in the model was 7.42 Å. This distance was obtained by relaxing the system starting from a variety of different initial structures. We set d.sub.N—C/Cu to take on values ranging from 6.42 Å to 9.42 Å, fixed the graphene layer, and allowed the other atoms to be relaxed. The stabilities of these systems are shown in Table 7. The results suggest that either decreasing or increasing the d.sub.N—C/Cu from 7.42 Å can lead to a decrease in stability, which suggest that 7.42 Å is the equilibrium distance between N—C layer and Cu layer due to the highest stability of the system.
[0181] We calculated the reaction energies of ethylene pathway (HOCCH* to CCH*) and ethanol pathways (HOCCH* to HOCHCH*) on N—C/Cu with the distances between N—C layer and Cu layer changing from 6.42 Å to 9.42 Å (Table 7). Similar to the results with the optimal d.sub.N—C/Cu (7.42 Å) for the main calculations, N—C/Cu still tends to improve the ethanol selectivity vs. ethylene with the d.sub.N—C/Cu range of 6.42 Å to 9.42 Å with respect to Cu (Table 2). Therefore, the calculation results seem to demonstrate that, ethanol selectivity can be promoted in N—C/Cu with the d.sub.N—C/Cu range of 6.42 Å to 9.42 Å, compared to bare Cu.
[0182] In addition, with the change in d.sub.N—C/Cu, the geometries of the key intermediate HOCCH*, the intermediate CCH*, and the intermediate HOCHCH* do not change significantly (
TABLE-US-00007 TABLE 7 Stabilities with respect to the ground state (Stability) and reaction energies of ethylene and ethanol pathways on N—C/Cu with different distances between N—C layer and Cu layer (d.sub.N—C/Cu). Energy (eV) d.sub.N—C/Cu Ethylene Ethanol E.sub.ethanol pathway − (Å) Stability pathway pathway E.sub.ethylene pathway 6.42 0.45 0.02 −0.24 −0.26 7.42 0.00 0.07 −0.03 −0.1 8.42 0.19 0.02 −0.01 −0.03 9.42 0.37 0.02 −0.11 −0.13
TABLE-US-00008 TABLE 8 Indexes, filenames of the xyz files in paper of Deringer et al. (ref. 80), indexes of the randomly doped nitrogen, number of carbon atoms, number of nitrogen atoms, and formation energies per atom (E.sub.f) of the top 10 most stable structures. Number of Number of carbon nitrogen E.sub.f Index Filename Index of the randomly doped nitrogen atoms atoms (eV) 1 DFT_relaxed_02_1.xyz 28, 6, 33, 1, 0, 23, 11, 18, 33, 18 27 6 0.07 2 DFT_relaxed_06_2.xyz 25, 13, 5, 20, 42, 26, 25, 15, 17, 12, 19, 33 32 9 0.11 3 DFT_relaxed_02_1.xyz 26, 22, 12, 34, 11, 1, 24, 17, 29, 3 25 8 0.12 4 DFT_relaxed_01_1.xyz 11, 12, 4, 25, 42, 36, 30, 36, 6, 22, 6, 40 33 8 0.13 5 DFT_relaxed_01_1.xyz 35, 36, 18, 27, 17, 8, 42, 9, 11, 35, 39, 15 32 9 0.14 6 DFT_relaxed_06_2.xyz 5, 16, 24, 3, 42, 2, 28, 13, 36, 10, 12, 12 32 9 0.14 7 DFT_relaxed_06_2.xyz 24, 9, 5, 7, 22, 33, 21, 24, 8, 15, 37, 36 32 9 0.18 8 DFT_relaxed_06_2.xyz 31, 10, 15, 39, 20, 23, 10, 40, 27, 11, 7, 4 32 9 0.18 9 DFT_relaxed_02_1.xyz 14, 1, 3, 27, 26, 34, 23, 28, 7, 19 25 8 0.18 10 DFT_relaxed_09_3.xyz 8, 24, 16, 11, 3, 9, 40, 28, 12, 15, 29, 28 30 9 0.20
TABLE-US-00009 TABLE 9 Reaction energies of ethanol and ethylene pathways on the top 10 most stable amorphous N-doped carbon structures. Reconstructed structures are labeled TRUE, while unreconstructed ones are labeled FALSE. Only the reaction energies of unreconstructed structures are used in the calculation of average reaction energies. All the energies are in eV. Ethanol pathway Ethylene pathway 1 −0.37 0.22 FALSE 2 0.05 0.01 FALSE 3 −0.35 0.22 FALSE 4 0.11 0.18 FALSE 5 −0.68 −0.15 FALSE 6 −1.01 −0.49 TRUE 7 −0.09 −0.78 FALSE 8 0.14 0.17 FALSE 9 0.28 0.18 FALSE 10 −2.02 0.09 TRUE average −0.12 0.01
TABLE-US-00010 TABLE 10 Product FEs for different nanocatalysts under different applied current densities in CO.sub.2RR. J.sub.total FE.sub.acetate FE.sub.ethanol FE.sub.ethylene FE.sub.n-propanol FE.sub.formate FE.sub.CO FE.sub.methane FE.sub.hydrogen Catalysts (mA cm.sup.−2) (%) (%) (%) (%) (%) (%) (%) (%) Cu 100 1.4 ± 0.4 17.7 ± 0.4 45.8 ± 2.5 5.6 ± 0.5 7.5 ± 1.1 6.2 ± 0.4 0.7 ± 0.1 11.9 ± 0.5 200 1.8 ± 0.3 24.9 ± 1.3 51.7 ± 0.7 4.5 ± 0.6 4.5 ± 0.9 2.8 ± 0.3 .sup. 1 ± 0.1 9.2 ± 0.5 300 2.3 ± 0.3 31.4 ± 2.7 48.2 ± 1.7 2.6 ± 0.2 1.8 ± 0.5 1.4 ± 0.3 1.7 ± 0.2 8.9 ± 0.3 34% 100 3.2 ± 0.3 27.5 ± 1.sup. 45.1 ± 3.4 7.7 ± 0.2 5.4 ± 0.8 2.9 ± 0.6 0.4 ± 0.1 6.6 ± 0.8 N—C/Cu 200 2.6 ± 0.2 36.1 ± 1.2 45.1 ± 1.7 3.7 ± 0.8 2.7 ± 0.1 1.2 ± 0.3 1.1 ± 0.1 5.6 ± 0.1 300 2.3 ± 0.8 52.3 ± 0.7 37.5 ± 0.5 1.4 ± 0.1 1.7 ± 0.4 0.3 ± 0.1 1.2 ± 0.2 7.4 ± 0.9 26% 100 2.7 ± 0.4 21.2 ± 0.9 46.2 ± 2.sup. 5.9 ± 1.sup. 7.5 ± 0.8 4.3 ± 0.4 0.6 ± 0.2 8.1 ± 0.3 N—C/Cu 200 2.4 ± 0.7 27.8 ± 1.5 52.2 ± 0.4 4.2 ± 0.8 2.3 ± 0.9 1.2 ± 0.4 .sup. 1 ± 0.1 .sup. 6 ± 0.2 300 .sup. 2 ± 0.5 42.6 ± 1.5 43.3 ± 1.5 0.9 ± 0.2 1.4 ± 0.2 0.6 ± 0.2 2.1 ± 0.8 7.8 ± 1.3 39% 100 1.9 ± 0.4 23.3 ± 0.9 48.8 ± 0.4 8.6 ± 1.9 5.1 ± 0.4 3.8 ± 0.7 0.7 ± 0.1 10 ± 0.3 N—C/Cu 200 3.1 ± 0.3 32.6 ± 0.5 46.7 ± 1.3 3.7 ± 0.3 2.9 ± 0.2 1.7 ± 0.3 0.7 ± 0.1 6.4 ± 0.1 300 3.6 ± 0.2 43.7 ± 0.8 44.2 ± 1.5 2.9 ± 0.4 2.4 ± 0.4 0.8 ± 0.1 2.5 ± 0.1 6.4 ± 0.1 C/Cu 100 1.8 ± 0.3 17.2 ± 1.1 41.8 ± 1.8 6.5 ± 1.5 16.3 ± 1.4 3.7 ± 0.6 0.5 7.4 ± 0.3 200 2.7 ± 0.1 25 ± 1 49.3 ± 0.8 2.2 ± 1.4 3.5 ± 0.3 1.5 ± 0.4 2.2 ± 0.2 8.6 ± 0.2 300 2.5 ± 0.3 30.5 ± 2.2 33.8 ± 1.6 1.8 ± 0.4 2.9 ± 0.3 0.6 ± 0.1 8.4 ± 0.3 20.5 ± 0.8
[0183] The rate-determining step of CO.sub.2RR is widely accepted as the first a few steps of CO.sub.2RR, including CO.sub.2 adsorption, CO.sub.2 activation, and *COOH hydrogenation (ref. 81). Chan, Nørskov, and co-workers (ref. 82) reported that after C—C coupling, the ensuing elementary steps are downhill. Therefore, the competing steps are HER and the first a few steps of CO.sub.2RR. To understand the effect of confinement on HER in N—C/Cu, we investigated CO.sub.2 adsorption, CO.sub.2 activation, *COOH hydrogenation, and reaction energies of (H.sup.++e.sup.− .fwdarw.* H) in HER on Cu and N—C/Cu (Table 11). The results suggest that, compared to the case of Cu, CO.sub.2 adsorption (CO.sub.2(g).fwdarw.*CO.sub.2) is enhanced due to the confinement effect of N—C/C. In contrast, the energies of other steps (*CO.sub.2.fwdarw.*COOH and *COOH.fwdarw.*CO) and reaction energies of (H.sup.++e.sup.−.fwdarw.* H) in HER on N—C/Cu are close to these on Cu. Therefore, introducing the N—C layer does not promote HER; its impact is instead to promote CO.sub.2RR activity. These results are consistent with the present experiment observation—H.sub.2 FEs decrease after introducing N—C layer on Cu (Table 10).
TABLE-US-00011 TABLE 11 CO.sub.2 adsorption energies (CO.sub.2(g) .fwdarw. *CO.sub.2), reaction energies of CO.sub.2 activation (*CO.sub.2 .fwdarw. *COOH), reaction energies of *COOH hydrogenation (*COOH .fwdarw. *CO), and reaction energies of (H.sup.+ + e.sup.− .fwdarw.* H) in HER on N—C/Cu and Cu. All the energies are in eV. N—C/Cu Cu CO.sub.2(g) .fwdarw. *CO.sub.2 0.60 1.17 *CO.sub.2 .fwdarw. *COOH 0.56 0.44 *COOH .fwdarw. *CO −0.50 −0.54 H.sup.+ + e.sup.− .fwdarw.* H −0.05 −0.06
TABLE-US-00012 TABLE 12 Cu.sub.ECSA determined by Pd.sub.UPD method for different samples. Cu.sub.ECSA (cm.sup.2) Cu 6.42 26% N—C/Cu 4.41 34% N—C/Cu 5.91 39% N—C/Cu 5.65
TABLE-US-00013 TABLE 13 Local pH at Cu surfaces for cases with and without the N—C layer. With N—C on Cu Only Cu Local pH 13.9 13.9
TABLE-US-00014 TABLE 14 Adsorption energies of CO (E.sub.CO) on pure Cu, C/Cu, and N—C/Cu. The adsorption energies are calculated with structures in FIG. 6 with two CO molecules, and the energies listed below are the average adsorption energies. E.sub.CO (eV) Cu −0.48 C/Cu 0.10 N—C/Cu −0.60
[0184] Supplementary Information about
[0185] The images used for the analysis of the gap are obtained with the camera not saturated in any regions of the field surveyed in these images (
[0186] Supplementary Information about
[0187] The resistance (R) between the working and reference electrodes with 34% N—C/Cu catalyst was measured in the course of constant-current (300 mA cm.sup.−2) electrolysis through electrochemical impedance spectroscopy (EIS) technique, and the corresponding iR drop was calculated (
6—Conclusions and Outlook
[0188] Confinement by covering an active copper-based electrocatalyst to enable molecules and solutions to intercalate was exploited to increase selectivity for ethanol. Density functional theory (DFT) calculations suggested that coating a nitrogen-doped carbon (N—C) layer on a Cu surface promotes C—C coupling and suppresses the breaking of the C—O bond in HOCCH*, thereby promoting ethanol selectivity in CO.sub.2RR. This was made possible by the strong electron-donating ability of the confining N—C layer. The catalyst delivered an ethanol FE of (52±1) % and an ethanol cathodic energy efficiency (EE) of 31%.
[0189] More particularly, this work thus showed how confinement effect arising due to an N—C layer on Cu catalysts, taken together with the strong electron-donating ability of the N—C layer, can enable advances in selectivity towards ethanol in CO.sub.2RR. An ethanol FE of (52±1) % with a partial ethanol current density (156±3) mA cm.sup.−2 on 34% N—C/Cu catalyst in CO.sub.2RR was observed. The cathodic EE and full-cell EE for ethanol also achieved a high value of 31% and 16%, respectively. These findings provide a route to improve the selectivity toward high-energy-density ethanol in CO.sub.2RR through catalyst design.
REFERENCES
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