BACKGROUND

1. Field

[0001] The present invention relates in general to a carbon dioxide (CO2) electrolysis operation mode, and more specifically to an improved CO2 reduction reaction (CO2RR) utilizing back pressure, and most specifically to a CO2RR electrochemical cell operation mode utilizing back pressure.

2. Description of the Related Art

[0002] CO2RR has many beneficial aspects. One aspect involves the clean generation of reaction products, such as hydrocarbons, alcohols, H2 and/or CO, that then can be used to feed any of a variety of industrial, commercial, individual or common inputs such as factories, gas turbines, equipment, vehicles, etc., as well as repositories and other storage mediums, as well as ambient or polluted air. Another aspect involves the consumption or destruction of greenhouse gas CO2, such as from CO2 emitting industrial, commercial, individual or common sources such as factories, gas turbines, equipment, vehicles, etc., as well as repositories and other storage mediums, as well as ambient or polluted air. Yet another aspect involves integration of CO2RR with renewable energy sources such as solar, wind, geothermal, etc., whereby the renewable energy sources provide the requisite electricity or E _o to initiate or sustain the CO2RR. Still another aspect involves combinations of two or more the above aspects, such as consuming CO2 flue gas from a factory source while sustaining the CO2RR from electricity generated from solar panels on the factory roof and then storing or feeding the produced value-added reaction product, such as ethene or higher order hydrocarbons, for a desired use.

[0003] Current CO2RR, electrochemical cells and operation modes, however, each suffer from one or more of a variety of problems, including difficulty of achieving product selectivity due to the narrow thermodynamic equilibrium potential (Eo) range between various reaction products such as:

[0004] at Eo [V v. SHE] of: -0.12

[0005] CO ₂ + H2O + 2e" -> CO + 2OH' at Eo [V v. SHE] of: -0.10

[0006] at Eo [V v. SHE] of: 0

[0007] CO2 + 5H ₂ O + 6e- ^CH ₃ OH + 6OH' at E _o [V v. SHE] of 0.02

[0008] at E _o [V v. SHE] of: 0.08

[0009] at Eo [V v. SHE] of: 0.09

[0010] of: 0.10

[0011] 2CO ₂ + 6H2O + 8e -^CHsCOOH + 8OH’ at E _o [V v. SHE] of 0. 11

[0012] CO2 + 6H2O + 8e’ at Eo [V v. SHE] of: 0.17

[0013] Other CO2RR, electrochemical cell and operation mode problems may include CO2 reaction inefficiency and instability overtime. Some efforts to overcome these problems involve modifying the catalyst surface such that the CO2 and its intermediary reactants are more strongly bounded on the catalyst. Other efforts to overcome these problems involve modifying the cathode to optimize gas diffusivity, porosity, thickness, hydrophilicity, electrical conductivity, ionic conductivity, wettability, etc. However, there remains a need for an improved CO2RR, electrochemical cell and/or operation mode and a particular need for an easily implemented, inexpensive and industrial scalable improved CO2RR, electrochemical cell, and/or operation mode. [0014]

SUMMARY

[0015] In an aspect of the invention, a CO2RR 2 is provided, comprising a three- phase boundary 14 having: a gaseous CO2 diffused at least partially through a porous cathode 8, a solid catalyst 10 operatively associated with the cathode 8, and a catholyte 12 or membrane 16 in communication with the catalyst 10; a pressure controller 30 adapted to control the pressure in gas chamber 6 and consequently the back pressure (pressure difference between the gas feed chamber 6 and the catholyte chamber 12) within, at or near the three-phase boundary 14; and a reaction product of the CO2RR such as but not limited to hydrocarbons, alcohols, H2 and/or CO.

[0016] In another aspect of the invention, an electrochemical cell 2 is provided, comprising: an inlet 4 through which gaseous CO2 enters the electrochemical cell 2 and advances into a chamber 6; a porous cathode 8 through which the gaseous CO2 in the chamber 6 can at least partially diffuse through; a solid catalyst 10 adhered to the cathode 8; a liquid catholyte 12 in fluid communication with the solid catalyst 10, wherein the gaseous CO2 contained in the porous cathode 8 and the solid catalyst 10 and the liquid catholyte 12 collectively form a three-phase boundary 14; a membrane 16 that separates the cathode 8 from an anode 20 to prevent short circuiting of the electrochemical cell 2 while allowing cations to circulate between the liquid catholyte 12 and a liquid anolyte 18; an outlet 22 through which reaction product(s) of the CO2RR exits the electrochemical cell 2; a pressure sensor 24 arranged within, at or near the three-phase boundary 14 to detect the pressure in catholyte chamber 12 within, at or near the three phase boundary 14; and a pressure controller 30 adapted to adjust the pressure in gas chamber 6 and consequently the back pressure (pressure difference between the gas feed chamber 6 and the catholyte chamber 12) within, at or near the three-phase boundary 14 . The pressure controller 30 maintains a pressure difference of 0 - 400 mbar and preferably 30 - 130 mbar between gas chamber 6 and catholyte chamber 12. [0017] In another aspect of the invention, a, CO2RR 2 is provided, comprising a three-phase boundary 14 having: a gaseous CO2 diffused at least partially through a porous cathode 8, a solid catalyst 10 operatively associated with the cathode 8, and a liquid catholyte 12 or membrane 16 in communication with the catalyst 10; a pressure controller 30 that controls the pressure within the three-phase boundary 14 in order to increase a dwell time of the CO2RR 2 within the three-phase boundary 14; and a reaction product of the CO2RR such as but not limited to hydrocarbons, alcohols, H2 and/or CO.

[0018] In another aspect of the invention, an electrochemical cell 2 is provided, comprising: an inlet 4 through which gaseous CO2 enters the electrochemical cell 2 and advances into a chamber 6; a porous cathode 8 through which the gaseous CO2 in the chamber 6 can at least partially diffuse through; a solid catalyst 10 adhered to the cathode 8; a liquid catholyte 12 in fluid communication with the solid catalyst 10, wherein the gaseous CO2 contained in the porous cathode 8 and the solid catalyst 10 and the liquid catholyte 12 collectively form a three-phase boundary 14; a membrane 16 that separates the cathode 8 from an anode 20 to prevent short circuiting of the electrochemical cell 2 while allowing cations to circulate between the liquid catholyte 12 and a liquid anolyte 18; an outlet 22 through which reaction product(s) of the CO2RR exits the electrochemical cell 2; pressure sensors 24, 26 arranged near the three-phase boundary 14 to detect the pressure within the three phase boundary 14; and a pressure controller 30 adapted to adjust the back pressure detected by the pressure sensors 24, 26 the pressure in gas chamber 6 and consequently the back pressure between 0 - 400 mbar in order to increase a dwell time of the CO2RR within the three-phase boundary 14.

[0019] These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention is shown in more detail by help of figures. The figures show preferred configurations and do not limit the scope of the invention.

[0021] FIG 1 is a detail view of a CO2RR three phase boundary utilizing back pressure in accordance with an exemplary embodiment of the subject matter.

[0022] FIG 2 is a schematic of a CO2RR electrochemical cell operation mode utilizing back pressure in accordance with an exemplary embodiment of the subject matter.

DETAILED DESCRIPTION

[0023] In the following detailed description of the present invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the subject matter or present invention.

[0024] As illustrated in Figure 1, a CO2RR utilizing back pressure to control CO2 dwell or occupation within, at or near a three-phase boundary and thereby control the CO2RR and desired reaction produces) is provided. The illustrated three-phase boundary comprises a gaseous CO2 diffused at least partially through a porous cathode 8, a solid catalyst 10 operatively associated with the porous cathode 8, and a liquid (or solid polymer) catholyte 12 or membrane 16 in communication with the catalyst 10. Establishing a back pressure, at or near the three-phase boundary 14 to 0 - 400 mbar and preferably to 30- 130 mbar is particularly suited for the efficient generation of hydrocarbon product(s). Reaction products may include hydrocarbons (of either or both higher and lower order), alcohols, H2 and/or CO. If no catholyte 12 is used, the catholyte 12 alkaline environment functionality may be achieved by membrane 16. Also in this catholyte-less configuration, the three- phase boundary 14 becomes the cathode 8, catalyst 10 and membrane 16, with membrane 16 liquidity source-able from the liquid anolyte 18 or other suitable liquid source associated with the cell 2. [0025] As illustrated in Figure 2, an operation mode utilizing back pressure on a CO2RR electrochemical cell 2 is also provided. The illustrated electrochemical cell 2 comprises a CO2 inlet 4, a three-phase boundary 14 comprising a porous cathode 8 through which gaseous CO2 is diffused, a solid catalyst 10 adhered to the cathode 8, a liquid catholyte 12 in fluid communication with the catalyst 10, as well as a membrane 16 that separates the cathode 8 from an anode 20 2 while allowing cations to circulate between the liquid catholyte 12 and a liquid analyte 18, along with a reaction product outlet 22. A pressure sensor 24 is arranged within, at or near the three-phase boundary 14 to detect the pressure in catholyte chamber 12 within, at or near the three-phase boundary 14. A pressure controller 30 is adapted to control the pressure in gas chamber 6 and consequently the pressure difference (back pressure) between gas chamber 6 and catholyte chamber 12. The pressure controller 30 maintains a pressure difference of 0 - 400 mbar and preferably 30 - 130 mbar between gas chamber 6 and catholyte chamber 12. However, as will be understood by those skilled in the art, depending on the desired CO2RR reaction product, particular electrochemical cell 2 configuration, three-phase boundary 14 constituents, and a variety of other variable factors such as gas diffusion layer constitution and operation conditions including applied current density, temperature, flow rates and the like, not all of the elements shown in Figure 2 are necessary to provide a CO2RR electrochemical cell 2 operation mode utilizing back pressure in order to generate CO2RR products.

[0026] Referring still to Figure 2, the electrochemical cell 2 has a CO2 inlet 4 configured to receive CO2 gas from any one or more of a variety of CO2 sources, including without limitation, industrial, commercial, individual or common sources such as factories, gas turbines, equipment, vehicles, etc., as well as repositories and other storage mediums, as well as ambient or polluted air. Depending on the desired reaction product, electrochemical cell 2 configuration, selected three-phase boundary 14 constituents, or a variety of other factors, the CO2 optionally may be conditioned before, within or after the inlet 4, such as by humidification via H2O or other suitable means. Figure 1 exemplarily illustrates humidified CO2 passing through the inlet 4 into a chamber 6 that holds and allows dispersion of the CO2. Of course, no or more than one inlet 4 and no or more than one chamber 12 can be used, and any inlet(s) 4 and chamber(s) 6 used can be arranged at different location(s) on, along, or through the electrochemical cell 2.

[0027] A porous cathode 8 is arranged to receive the gaseous CO2 and configured such that the CO2 can diffuse through at least a portion of the cathode 8. The exemplarily illustrated cathode 8 is a gas diffusion electrode (GDE) that absorbs and converts the CO2 molecules into the desired CO2RR reaction product e.g. hydrocarbons, however other suitable cathodes 8 could be used. A solid catalyst 10 is operatively associated with the cathode 8 by any suitable means, such as drop casting, plating, doping, or spray coating to enhance reaction product selectivity as well as reaction efficiency and stability. Suitable catalysts 10 include but are not limited to Pt, Zn, Cu, Ag, Au, Pd and Sn. Since Cu is the only transition metal catalyst for CO2RR to value added C2+ reaction products e.g. ethene, ethanol, propanol, Cu is therefore preferred but not required when desiring those reaction products.

[0028] A catholyte 12 is advantageously arranged in communication with the catalyst 10 to provide an alkaline environment close to the three-phase boundary 14 and thereby promoting CO2RR thermodynamically and kinetically when hydrocarbon reaction product(s) and/or a CO reaction product is desired. The exemplarily illustrated catholyte 12 is an alkaline buffer liquid solution, such as potassium hydrogen carbonate, cesium hydrogen carbonate, rubidium hydrogen carbonate, lithium bicarbonate or potassium hydroxide, but the catholyte 12 may also be embodied as a solid polymer.

[0029] A three-phase boundary 14 is thereby formed by: (1) the gaseous CO2 diffused at least partially through the cathode 8, (2) the solid catalyst 10 operatively associated with the cathode 8, and (3) the liquid (or solid polymer) catholyte 12 in communication with the catalyst 10. Figure 1 provides a detailed illustration of the three- phase boundary, exemplary H+ and CO2 reaction constituents, as well as exemplary desired and undesired CO2RR reaction products.

[0030] Depending on the desired reaction product(s), electrochemical cell 2 configuration, selected three-phase boundary 14 constituents, or a variety of other variable factors such as gas diffusion layer constitution and operation conditions including applied current density, temperature, flow rates and the like, optionally, a membrane 16 optionally may be used to separate the cathode 8 from an anode 20 while allowing cations to circulate between the liquid catholyte 12 and a liquid anolyte 18. Exemplary suitable membranes 16 include sulfonated tetrafluoroethylene based fluoropolymer-copolymer (Nafion). If used, the anolyte 18 is advantageously an alkaline buffer solution to complete the ion circuitry of the electrochemical cell 2. Exemplary suitable analytes 18 include H2O, OH-, H ⁺ and electrolyte associated cations and anions (e.g. Cs” and SOfi ² ). Also if used, the anode 60 material can be composed of Ir, Ni and/or Pt, where the anode would be responsible for O2 evolving reaction and completing the ion circuit in the electrochemical cell by pumping H ⁺ to the cathode.

[0031] Similarly depending on the desired reaction product(s), electrochemical cell 2 configuration, selected three-phase boundary 14 constituents, or a variety of other variable factors such as gas diffusion layer constitution and operation conditions including applied current density, temperature, flow rates and the like, optionally, the membrane 16, preferably an anion exchange membrane 16 or a bipolar membrane 16, may be used to sustain the alkaline environment rather than or in addition to the catholyte 12. In this configuration, no catholyte 12 is used and the catholyte 12 alkaline environment functionality is achieved by membrane 16. Also in this catholyte-less configuration, the three-phase boundary 14 becomes the cathode 8, catalyst 10 and membrane 16, with membrane 16 liquidity source-able from the liquid anolyte 18 or other suitable liquid source associated with the cell 2.

[0032] A reaction product outlet 22 is configured to receive the reaction product, e.g. hydrocarbon gas, from the electrochemical cell 2 and feed the reaction product to any one or more use sources, including without limitation, industrial, commercial, individual or common sources such as factories, gas turbines, equipment, vehicles, etc., as well as repositories and other storage mediums, as well as ambient or polluted air. Depending on the desired reaction product, electrochemical cell configuration 2, selected three-phase boundary 14 constituents, or a variety of other factors such as reaction product use source(s), the CO2 reaction product optionally may be conditioned before, within or after the outlet 22, such as by liquifying (e.g. to form LNG particularly if methane is a reaction product). Unconditioned reaction products are exemplary shown as passing through the outlet 22. Of course, no or more than one outlet 22 can be used, and any outlet(s) 22 used can be arranged at different location(s) on, along, or through the electrochemical cell 2.

[0033] Still referring to Figure 2, a pressure sensor 24 is arranged in operative communication with the electrochemical cell 2 in order to detect pressure in catholyte chamber 12 within, at or near the three-phase boundary 14. The exemplary embodiment shows the pressure sensor 24 arranged near the three-phase boundary 14, toward the middle of the catholyte 12 between the cathode 8 and membrane 16, however, depending on the desired reaction product(s), electrochemical cell 2 configuration, three-phase boundary 14 constituents, or a variety of other factors, the pressure sensor 24 may be arranged in any of a variety of locations within, at or near the three-phase boundary 14. Also, if the three- phase boundary 14 is expansive or geometrically complex, then pressure sensors 26, 28 advantageously may be used to better sense the three-phase boundary 14 pressure and control the CO2RR.

[0034] An outlet pressure sensor 26 may be advantageously arranged within, at or near the outlet 22 in order to detect pressure within, at or near the outlet 22. The exemplary illustration shows an outlet pressure sensor 26 arranged at the beginning of the outlet 22. Of course, depending on the outlet 22 configuration, one or more optional outlet pressure sensors 26 could be arranged in any of a variety of locations within, at or near the outlet 22 to better sense the outlet pressure and control the CO2RR. Also optionally, an inlet pressure sensor 28 may be arranged within, at or near the inlet 4 in order to detect CO2 pressure within, at or near the inlet 4. The exemplary illustration shows an optional inlet pressure sensor 28 arranged at the end of the inlet 4. Of course, depending on the inlet configuration, one or more optional outlet pressure sensors 28 could be arranged in any of a variety of locations within, at or near the inlet 4 to better sense the inlet pressure and control the CO2RR.

[0035] A pressure controller 30 is arranged in operative communication with the pressure sensor(s) 24, 26 and/or optional pressure sensor 28 and is adapted to adjust pressure detected by the pressure sensor(s) 24, 26 and/or optional pressure sensor 28 in order to adjust the back pressure within, at or near the three phase boundary 14 and/or outlet 22 or inlet 4. The exemplary illustration shows the pressure controller 30 located downstream of the outlet 22 and outlet pressure sensor 26 and embodied as a membrane valve with a flexible diaphragm that allows outlet flow only at, above, below or between a desired pressure. Of course, depending on the desired reaction product(s), electrochemical cell 2 configuration, three-phase boundary 14 constituents, and a variety of other factors such as desired pressure(s), etc., the pressure controller 30 may be embodied through any of a variety of suitable mechanisms and may be arranged in any of a variety of locations. Also, if the electrochemical cell 2, CO2 feed source, or reaction product feed use is expansive or complex, then additional pressure sensors 30 may be advantageously used to better control and control the CO2RR.

[0036] Still referring to Figure 2, an exemplary illustration of CO2RR electrochemical cell 2 operation mode utilizing back pressure is provided. Pressurized humidified gaseous CO2 enters electrochemical cell 2 through gas inlet 4 and into chamber 6 in the range of 95 - 10 vol %, preferably 80 - 30 vol %, in AP = 0 - 400 mbar, preferably 50-200 mbar and Vco2 = 5 - 200 seem, preferably 15-50 seem, The CO2 is urged toward cathode 8 and diffuses through at least a portion of cathode 8, thereby contacting catalyst 10. The CO2RR thereby occurs within, at or near the three-phase boundary 14, with the reaction product then advancing through the outlet 22 for desired collection or use. Reaction product selectivity, as well as efficiency and stability, is improved by controlling the CO2 pressure within, at or near the three-phase boundary 14. One way to control the CO2 pressure within, at or near the three-phase boundary 14 is via the pressure controller 30 arranged in operative communication with the pressure sensor(s) 24 and 26, and/or optional pressure sensor 28 to thereby adjust pressures detected by the pressure sensor(s) 24 and 26, and/or optional pressure sensor 28 in order to adjust the back pressure within, at or near the three-phase boundary 14 and/or outlet 22 or inlet 4.

[0037] In an exemplary CO2RR operation, CO2 inlet 4 pressure (Pl) is sensed as atmospheric at 1 mbar (within an exemplary preferred range of 0 mbar to 10 mbar), while three-phase boundary 14 pressure (P2) is sensed at 20 mbar (outside an exemplary preferred range of 30 mbar to 130 mbar). Based on the P2 and Pl pressure difference and the preferred pressure ranges, the three-phase boundary 14 pressure (P2) is adjusted to a pressure of 85 mbar (or anywhere within the exemplary preferred range of 30 mbar to 130 mbar) in order to better control the CO2RR and desired reaction product.

[0038] In a second exemplary operation, CO2 outlet 22 pressure (P3) is 15 mbar (within an exemplary preferred range of 10 mbar to 30 mbar), while three-phase boundary 14 pressure (P2) varies between 120-150 mbar (partially within and partially outside an exemplary preferred range of 30 mbar to 130 mbar). Based on the P2 and P3 pressure difference and the preferred pressure ranges, the three-phase boundary 14 pressure (P2) is continually adjusted to a desired set pressure of 70 mbar (or anywhere within the exemplary preferred range of 30 mbar and 130 mbar) in order to better control the CO2RR and desired reaction products.

[0039] In a third exemplary operation, CO2 inlet 4 pressure (Pl) is 2-4 mbar as continually measured by inlet pressure sensor 28, while three-phase boundary 14 pressure (P2) is 60-75 mbar as continually measures by pressure sensor 24, and outlet 22 pressure (P3) is 5-10 mbar as continually measured by outlet pressure sensor 26. Based on the P3, P2 and Pl pressure differences, as well as an exemplarily desired three-phase boundary 14 pressure range of 0 - 400 mbar (P3), an exemplarily desired inlet 4 pressure of 1 mbar (Pl) and an exemplarily desired outlet 22 pressure range of 2-5 mbar (P3), controller 30 periodically adjusts the three-phase boundary 14 pressure (P2) to 55 mbar (or anywhere between 0 mbar and 400 mbar), the inlet 4 pressure (Pl) to 1 mbar and the outlet 22 pressure (P3) to 4 mbar, in order to better control the CO2RR and desired reaction products. [0040] In a fourth exemplary operation, CO2 inlet 4 pressure (Pl) is 50-75 mbar as continually measured by inlet pressure sensor 28, while three-phase boundary 14 pressure (P2) is 150-180 mbar as continually measures by pressure sensor 24, and outlet 22 pressure (P3) is 250-270 mbar as continually measured by outlet pressure sensor 26. Based on the P3, P2 and Pl pressure differences, as well as an exemplarily desired three- phase boundary 14 pressure range of 0-400 mbar (P3), an exemplarily desired inlet 4 pressure of 75-100 mbar (Pl) and an exemplarily desired outlet 22 pressure range of 300- 320 mbar (P3), controller 30 periodically adjusts the three-phase boundary 14 pressure (P2) to 175 mbar (or anywhere between 80-100), the inlet 4 pressure (Pl) to 100 mbar and the outlet 22 pressure (P3) to 250 mbar, in order to better control the CO2RR and desired reaction products.

[0041] In a fifth exemplary operation, a plurality of inlet pressure sensors 28 are employed near inlet 4 from which an average pressure (Pl) of 2 mbar is calculated, while a plurality of outlet pressure sensors 26 are employed near outlet 22 from which an average pressure (P3) of 10 mbar is calculated. Based on the P3 and Pl pressure difference, as well as an exemplarily desired three-phase boundary 14 pressure range of 30 - 130 mbar, an exemplarily desired inlet pressure (Pl) of 1 mbar and an exemplarily desired outlet pressure range (P3) of 2-5 mbar, controller 30 adjusts the inlet pressure (Pl) to 1 mbar and the outlet pressure (P3) to 3 mbar, in order to better control the CO2RR and desired reaction products. In this exemplary operation, pressure sensor 24 is not used while pressure sensors 26 and 28 are used.

[0042] Through the above utilization of back pressure with CO2RR or with a CO2RR electrochemical cell 2 operation mode, CO2 availability at the three-phase boundary 14 can be increased. It has been observed that increased CO2 availability suppresses availability of undesired constituents such as those forming hydrogen evolving reaction (HER) reaction products, enhances the desired CO2RR product(s), and helps prevent the liquid catholyte 12 from flooding the catalyst 10 or cathode 8. Thus, the above utilization of back pressure with CO2RR or with a CO2RR electrochemical cell 2 operation mode thereby overcomes difficulty of achieving reaction product selectivity as well as CO2RR reaction inefficiency and instability over time. The need for an improved CO2RR and the need for an easily implemented, inexpensive and industrial scalable CO2RR electrochemical cell 2 operation mode is thereby provided through the above selective utilization of back pressure with CO2RR electrochemical cell 2 operation.

[0043] While specific exemplary embodiments and illustrations have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the subject matter, which is to be given the full breadth of the appended claims, and any and all equivalents thereof.