A CARBON-EFFICIENT BICARBONATE ELECTROLYZER
Described herein is a carbon-efficient bicarbonate electrolyzer and assembly. In some embodiments, an electrolyzer assembly comprises: a cathode disposed in a cathode chamber having ports for receiving a bicarbonate (HCO3−) solution; an anode disposed in an anode chamber having ports for receiving an anolyte; a cation exchange membrane disposed between the cathode chamber and the anode chamber; and a buffer layer disposed between the cathode chamber and the cation exchange membrane. In some embodiments, the electrolyzer assembly is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
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This application claims the benefit under 35 U.S.C. § 119 of U.S. Provisional Patent Application No. 63/493,400 filed on Mar. 31, 2023, which is hereby incorporated by reference herein in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCHThis invention was made with government support under DE-SC0023450 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
BACKGROUNDCarbon efficiency is one of the most pressing problems of carbon dioxide (CO2) electroreduction (CO2ER) today. CO2ER is an attractive strategy of CO2 utilization for providing various chemicals (e.g., formic acid (HCOOH), carbon monoxide (CO), ethylene (C2H4), acetic acid (CH3COOH), ethanol (C2HOH), etc.) while simultaneously decreasing atmospheric CO2.
SUMMARYConventionally, CO2 capture and CO2ER are considered separately. Typically, CO2 captured from dilute sources is converted (typically to solid calcium carbonate (CaCO3)), transported, thermally decomposed, and pressurized to become an ultra-pure CO2 gas solution. For the most prevalent alkaline or neutral medium based aqueous membrane electrode assembly (MEA) electrolyzers, over 50% of the capital cost would come from this CO2 thermal regeneration process, which can be energy intensive. Further, in an aqueous environment, CO2ER involves proton-coupled electron transfer (PCET) reactions, locally generating hydroxides (e.g., CO2+H2O+2e−→CO+2OH−). The OH− would have a chance to trap reactive CO2* into electrochemically inert CO32−(aq) (2OH−+CO2→CO32−), which is a “trap state”.
In a CO2 membrane electrode assemblies (MEA) electrolyzer with an anion exchange membrane (AEM), HCO3−/CO32− are the major charge carriers in the electrolyte and would cross over the membrane to the anode side with a high chance of being wasted. This carbon efficiency loss problem is more detrimental to products with more electrons transferred. For instance, for the 2-electron-transfer reaction of CO production, 2 OH− are generated per 1 CO2 converted, and thus the carbon efficiency loss is 50%. For the 6-electron-transfer reaction of C2H5OH production, 6 OH− are generated per 1 CO2 converted and thus the efficiency loss is 75%. Further, the parasitic hydrogen evolution reaction (HER) also produces OH− (2H2O+2e−→H2+2OH−), which further lowers the actual carbon efficiency and faradaic efficiency (FE). Conventional CO2ER research accounts for a percentage of CO2 converted, but leave out a substantial amount of CO2 that was unconverted or wasted as said amount is either lost as CO32−/HCO3− or simply unreacted and merges into a gaseous mixture with the outlet products. Thus, the combined carbon efficiency and energy efficiency of CO2 capture and electroreduction, through the traditional route with solid CaCO3 and ultra-pure gaseous CO2, is low because of the low CO2ER yield.
According to one aspect of the present disclosure, an electrolyzer comprises: a cathode configured to receive a bicarbonate (HCO3−) solution; an anode configured to receive an anolyte; and a cation exchange membrane disposed between the cathode and the anode, wherein the electrolyzer is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
In some embodiments, the bicarbonate (HCO3−) solution is potassium bicarbonate, sodium bicarbonate, or calcium bicarbonate. In some embodiments, the formate (OOCH−) solution is potassium formate, sodium formate, or calcium formate. In some embodiments, the bicarbonate (HCO3−) solution is a liquid and the formate (OOCH−) solution is a liquid or a mixture of a solid and a liquid. In some embodiments, the bicarbonate (HCO3−) solution is provided within a cathode chamber of the cathode, wherein the cathode chamber has a pressurized gas phase in a headspace of the cathode chamber sealed from an external air, wherein a pressure in the cathode chamber is controlled by a CO2 gas cylinder to between 0.01 MPa and 30 Mpa. In some embodiments, the cathode chamber has a CO2 gas in the headspace of the cathode chamber, wherein the CO2 gas cylinder can control a partial pressure of the CO2 gas to between 0.001 Mpa and 30 Mpa. In some embodiments, a temperature in the cathode can be controlled using at least a heating pad to between 0 degrees Celsius and 300 degrees Celsius. In some embodiments, the anolyte has a specific initial pH between 5 and 7 and the bicarbonate (HCO3−) solution has a specific initial pH between 6 and 9. In some embodiments, the cathode includes a substrate loaded with tin nanoparticles. In some embodiments, a buffer layer is disposed between the cathode and the cation exchange membrane, wherein the buffer layer has one or more glass fibers impregnated with a bicarbonate.
According to another aspect of the disclosure, an electrolyzer assembly comprises: a cathode disposed in a cathode chamber having ports for receiving a bicarbonate (HCO3−) solution; an anode disposed in an anode chamber having ports for receiving an anolyte; a cation exchange membrane disposed between the cathode chamber and the anode chamber; and a buffer layer disposed between the cathode chamber and the cation exchange membrane, wherein the electrolyzer assembly is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
In some embodiments, the bicarbonate (HCO3−) solution is potassium bicarbonate, sodium bicarbonate, or calcium bicarbonate. In some embodiments, the bicarbonate (HCO3−) solution is a liquid. In some embodiments, the formate (OOCH−) solution is potassium formate, sodium formate, or calcium formate. In some embodiments, the formate (OOCH−) solution is a liquid or a mixture of a solid and a liquid. In some embodiments, the cathode chamber has a pressurized gas phase in a headspace of the cathode chamber sealed from an external air, wherein a pressure in the cathode chamber can be controlled by a CO2 gas cylinder to between 0.01 MPa and 30 MPa. In some embodiments, the cathode chamber has a CO2 gas in the headspace of the cathode chamber, wherein the CO2 gas cylinder controls a partial pressure of the CO2 gas to between 0.001 MPa and 30 MPa. In some embodiments, a temperature in the cathode is controlled using at least a heating pad to between 0 degrees Celsius and 300 degrees Celsius. In some embodiments, the anolyte has a specific initial pH between 5 and 7 and the bicarbonate (HCO3−) solution has a specific initial pH between 6 and 9.
According to another aspect of the disclosure, an electrolyzer assembly comprises: a cathode disposed in a cathode chamber having ports for receiving a bicarbonate (HCO3−) solution; wherein the cathode chamber has a pressurized gas phase in a headspace of the cathode chamber sealed from an external air with a pressure in the cathode chamber controlled by a CO2 gas cylinder to between 0.01 MPa and 30 MPa, wherein a temperature in the cathode chamber is controlled using at least a heating pad to between 0 degrees Celsius and 300 degrees Celsius; an anode disposed in an anode chamber having ports for receiving an anolyte; a cation exchange membrane disposed between the cathode chamber and the anode chamber; and a buffer layer disposed between the cathode chamber and the cation exchange membrane, wherein the electrolyzer assembly is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
In some embodiments, the bicarbonate (HCO3−) solution is potassium bicarbonate, sodium bicarbonate, or calcium bicarbonate. In some embodiments, the bicarbonate (HCO3−) solution is a liquid. In some embodiments, the formate (OOCH−) solution is potassium formate, sodium formate, or calcium formate. In some embodiments, the formate (OOCH−) solution is a liquid or a mixture of a solid and a liquid. In some embodiments, the cathode chamber has a CO2 gas in the headspace of the cathode chamber, wherein the CO2 gas cylinder controls a partial pressure of the CO2 gas to between 0.001 MPa and 30 MPa. In some embodiments, the anolyte has a specific initial pH between 5 and 7 and the bicarbonate (HCO3−) solution has a specific initial pH between 6 and 9.
The manner and process of making and using the disclosed embodiments may be appreciated by reference to the figures of the accompanying drawings. It should be appreciated that the components and structures illustrated in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the concepts described herein. Like reference numerals designate corresponding parts throughout the different views. Furthermore, embodiments are illustrated by way of example and not limitation in the figures, in which:
Turning to
Anode 110 comprises an anode flow plate 112 positioned away from CEM 130 to allow for containment of liquid anolyte 114 therebetween. Anolyte 114 can be provided as a water anolyte (H2O) and, more particularly, a near-neutral water anolyte. The near-neutral water anolyte can have a specific initial pH in the range 5 to 7, with local pH near anode even more acidic. In some cases, anolyte 114 can be provided as pure water. In some cases, to provide a more acidic anolyte 114, anolyte 114 can be provided as sulfuric acid mixed with water.
Cathode 120 comprises a cathode flow plate 122 positioned away from CEM 130 to allow for containment of a liquid catholyte 124 therebetween. Catholyte 124 can be provided as a bicarbonate solution (or “feedstock”), such as potassium bicarbonate, sodium bicarbonate, calcium bicarbonate, or any aqueous solution containing HCO3−(aq). Catholyte 124 can have a specific initial pH in the range 6 to 9, for example.
Thus, electrolyzer 100 can have a variable pH or a constant pH across the cell. Variable pH means that the electrolyzer 100 has a gradient of pH from the cathode 120 to the anode 110; this may be an initial condition that can change over time, e.g., during electrolysis. The anolyte 114 can have a specific initial pH (e.g., in the range of 5 to 7, with the pH near the anode more acidic) different from the specific initial pH of the catholyte 124 (e.g., in the range of 6 to 9) during an initial time period (initial time period referring to the time wherein the electrolyzer is setup and/or during the start of electrolysis). As electrolysis progresses, the pH across the electrolyzer can reach equilibrium such that the anolyte 114 and catholyte 124 have essentially the same pH. For example, the anolyte 114 and catholyte 124 may both have a pH of 6 or 7.
Anode 110 and cathode 120 can be provided within separate sealed chambers (such as shown in
CEM 130 may be provided as a material that permits the exchange of cations between anode 110 and cathode 120. CEM 130 may be configured (e.g., using a water anolyte) to shuttle protons from anode 110 to the cathode 120 to complete an internal circuit. CEM 130 may be provided having a near-neutral-pH (pH in the range 5 to 7). The CEM 130, for example, may be a Aquivion E98-09S with a thickness of 90 μm. The CEM 130 may, for example, comprise NAFION or a similar material.
Buffer layer 140 can include glass fibers (e.g., artificial glass fibers) and bicarbonate, for example. In some cases, buffer layer 140 can be impregnated with bicarbonate. Buffer layer 140 can be configured to neutralize protons shuttling directly from the anode side, reducing over-acidity at the cathode/electrolyte surface, thus reducing (and ideally preventing) HER. This can be especially important at high current densities (e.g., current densities larger than 1 A/cm2) when the proton flux is high. The current density for the electrolyzer 100 may be moderate (e.g., about 0.1 A/cm2). Buffer layer 140 may also enhance the FE by inducing a pH gradient in the electrolyzer 100. Through the positioning of the buffer layer 140 between the cathode 120 and CEM 130, the buffer layer 140 acts to buffer pH in the electrolyzer 100, for example to ensure the surface of the cathode 120 is not too acidic. For example, the buffer layer 140 may be prepared by immersing a WHATMAN glass fiber in a bicarbonate solution. The WHATMAN glass fiber may be WHATMAN GF/D microfiber filters with a diameter of 35 mm. The bicarbonate solution, for example, may have the same bicarbonate concentration as the bicarbonate solution provided as the catholyte 124.
One or more tin nanoparticles are loaded or otherwise disposed on the surface of the cathode 120. The tin nanoparticles may be tin nanoparticle catalysts. For example, to prepare the tin (Sn) electrodes, a mixture of tin nanoparticles, multi-walled carbon nanotubes, and NAFION 117 is prepared with a weight ratio of 1:0.2:1 using pure ethanol. The NAFION ionomer, for example, may have a 5 wt. %. Accordingly, the Sn/NAFION weight ratio is 20:1 rather than 1:1. The loading density for the Sn electrodes may, for example, be about 4 mg/cm2 (+/−1 mg/cm2).
In operation, electrolyzer 100 can convert bicarbonate solution to a formate (OOCH−) solution. Buffer layer 140 and CEM 130 function to separate the varying pH in between anode 110 and cathode 120. Accordingly, the variable pH across the electrolyzer 100 results in a near-neutral-pH (near-neutral meaning a pH in the range 5 to 7, with local pH near the anode 110 more acidic) CEM 130.
As previously discussed, the anolyte 114 and catholyte 124 can be contained within separate sealed chambers. The pressure in one or both chambers may be controlled to improve performance. For example, a gas phase sealed in the headspace of the cathode chamber, which is sealed off from external air and pressurized, can be controlled to have a CO2(gas) partial pressure ranging between 0.001 Mpa and 30 Mpa, wherein the total pressure ranges between 0.01 Mpa and 30 Mpa. In order to control the pressure, the cathode chamber can be connected to a CO2 gas cylinder. In some cases, CO2 partial pressure management may be used. The CO2 gas cylinder can control the partial pressure of the CO2 gas in the cathode chamber. The anode chamber may not be pressurized.
The temperature of electrolyzer 100 may also be controlled to improve performance. Temperature management can provide a number of benefits. First, temperature impacts the solubility of bicarbonate salt in the solution. A high temperature (e.g., higher than room temperature or, in some cases, 90° C. to 110° C.) leads to higher solubility of bicarbonate, as the solubility of potassium bicarbonate at 100° C. is around twice that at room temperature (e.g., a temperature between 20° C. to 25° C.). Second, temperature influences the CO2 exsolution rate from the solution. A high temperature expedites the CO2 outgassing from the water in the anode 110, decreasing the amount of CO2 available for the reaction, and may adversely impact the carbon efficiency. Accordingly, there is a trade-off in terms of temperature effects. In some examples, the cathode chamber can be controlled to have a temperature between 0° C. and 300° C. The cathode chamber can be controlled to have a temperature between room temperature and 80° C. The cathode chamber can be controlled to have a temperature to 300° C. with the application of a high CO2 pressure. To control the temperature, the electrolyzer 100 can be disposed on a heating pad and the respective liquids 114 and 124 can be disposed on a heating stage.
By controlling pH, pressure, and/or temperature in the electrolyzer 100, and by the inclusion of the CEM 130 and buffer layer 140, aqueous bicarbonate solution can be converted into solid formate fuel with a high carbon efficiency, e.g., a formate yield of about 96% (+/−4%). Thus, electrolyzer 100 can realize complete (or near-complete) yield of formate from bicarbonate with elimination of carbon efficiency loss.
The control of pH in the electrochemical cell is notably useful, as H+(aq) is a promoter of CO2ER. During operation of the electrolyzer 100, reactions that lead to net base production, such as CO production from HCO3− (the full reaction: HCO3−→CO+½O2+OH−), would create excess deleterious CO32−(aq) and OH−, buffering the solution and impeding its further operation. Accordingly, the conversion of bicarbonate would be restricted. Consequently, the selectivity of CO2ER would decline continuously when an electrolyzer 100 is operated in a constant-current mode (e.g., meaning a mode of operation where a constant current flows through the electrolyzer).
The formate solution converted in the cathode 120 may be a liquid or a solid-liquid mixture. The formate solution may be potassium formate, sodium formate, calcium formate, or a mixture of formate containing OOCH−(aq). Accordingly, an example of the full-cell reaction achieved is 2KHCO3→2KOOCH+O2, with no net acid or base produced. No net acid or base production is notably useful in maintaining a steady pH and high yield (e.g., a formate yield of about 96% (+/−4%)).
Electrolyzer 100 can be configured to have a high concentration of HCO3−(aq) that is maintained while CO32−(aq) is excluded in the cathode 120 through control of at least one variable. The at least one variable can include, for example: the pH of both the cathode and anode; a CO2(gas) partial pressure in the cathode chamber; and a temperature in the cathode chamber. A high carbon efficiency or a high yield of the conversion of bicarbonate to formate can be achieved partially because of an excess of HCO3−(aq) in the cathode and a lack of CO32−(aq).
Anode chamber 156 can include two ports 160, 162 to which two tubes 164, 166 can be respectively connected. Likewise, cathode chamber 158 can include two ports (not visible in the figure) to which two tubes 168, 170 can be respectively connected. The tubes 164, 166, 168, 170 may be made of silicon, for example.
Cathode chamber tubes 168, 170 can be used to circulate catholyte (e.g., liquid bicarbonate) in the cathode, as well as to inject gas (e.g., CO2) to control pressurization in the cathode chamber 158. The cathode chamber tubes 168, 170 can connect to the CO2 cylinder to provide gas and control pressure. In more detail, tubes 168, 170 can be connected to a pump (e.g., a peristaltic pump) with a constant flow rate of about 10 mL/min (+/−1 mL/min). The same catholyte electrolyte solutions may be recirculated in and out of a bulk solution to form a closed loop. Deionized water (18.2 MO) may, for example, be used to prepare the catholyte electrolyte. The cathode chamber tubes 168, 170 may be a multi-walled carbon nanotube.
One or more carbon substrates may be used as the cathode substrate. The tin nanoparticles are loaded onto the carbon substrate. The carbon substrates (e.g., AVCARB 190 carbon paper or PANEX carbon cloth) may be hydrophobic carbon substrates. The hydrophilic carbon substrates, for example, are prepared by immersing the carbon substrates in a concentrated nitric acid (70%) overnight (e.g., for about 12 hours). The cathode electrodes may, for example, be prepared (or loaded) by airbrushing precursor solutions onto the carbon substrates.
Anode chamber tubes 164, 166 can be used to circulate anolyte (e.g., pure water) in the anode, enabling electrolysis. In more detail, tubes 164, 166 can be connected to a pump (e.g., a peristaltic pump) with a constant flow rate of about 10 mL/min (+/−1 mL/min). The same anolyte electrolyte solutions may be recirculated in and out of a bulk solution to form a closed loop. Deionized water (18.2 MO) may, for example, be used to prepare the anolyte electrolyte. The anode chamber tubes 164, 166 may be a multi-walled carbon nanotube. The anode may, for example, be provided in part as IrO2 on carbon paper.
Anode chamber 156 includes an anode terminal 172 to which a first wire 176 can be connected. Cathode chamber 158 includes a cathode terminal 174 to which a second wire 178 can be connected. Internal to the chambers, anode terminal 172 can be connected to an anode flow plate (e.g., plate 112 of
According to a first pathway, at step 182, flue gas or air can be captured. Next, at step 184, CO2 solution can be captured from said flue gas or air. Once captured, CO2R may occur through the CO2 solution with a solution of K2CO3 (aq). At step 186, the K2CO3 can be processed via a pellet reaction and calcination. Accordingly, the chemical reaction may occur as follows: CaCo3 (s)→CaO (s)+CO2 (g), with an enthalpy change of 178.3 kJ/mol. At step 188, the gas can be compressed to produce pure CO2 (g). At step 190, the pure CO2 (g) can undergo electrolysis.
According to a second pathway, after capture of the CO2 solution (step 184), CO2R may occur via a bicarbonate solution KHCO3 (aq). Then, at step 190, the bicarbonate solution KHCO3 (aq) can undergo electrolysis.
In a bicarbonate electrolyzer, the electrochemical CO2ER is preceded by in situ CO2* generation from HCO3-(aq) as:
xHCO3−(aq)+xH+(aq)→xCO2*+xH2O(aq)
xCO2*+ze−+nH+(aq)→(CxHn-2yO2x-y)n-z(aq)+yH2O(aq)
In either pathway, the electrolysis step 190 can produce a gas stream and a liquid stream. The gas stream may include one or more of: CO2; CO; H2; C3H4; CH4; etc. The liquid stream may include one or more of: HCOOH; CH3OH; C2H5OH; CH3COOH; etc. The decoupling of the carbon capture and CO2R processes and the demand for pure CO2 solution calls for an energy-intensive thermal regeneration process.
A bicarbonate solution (KHCO3 (aq)) can be fed into the cathode 220 (as indicated by arrow 240) and the following reaction can occur:
H++HCO3−→H2O+CO2* (which consumes 1H+)
CO2*+2e−+H+→HCOO− (which consumes 1H+)
2H+ is transferred over the CEM 230. The bicarbonate solution ensures proton balance and the KHCO3-based formate production enables pH balance.
Although CO2(aq), HCO3−(aq), CO32−(aq) are often considered to be in the same family, their carbon centers are different in terms of electrophilicity and thus, the microscopic reaction pathways are different. In order for electroreduction to occur, CO2(aq)/HCO3−(aq)/CO32−(aq) are first converted into CO2*, with the surface-adsorbed intermediate at the electrocatalyst/aqueous solution interface before electron transfer from the cathode 220 can occur (a tin nanoparticle catalyst may be utilized, for example tin nanoparticles with a size of 60-80 nm). Using a CEM, the production of formate from pure HCO3−(aq) solution does not alter the catholyte pH. This is because for every formate molecule generated, 2 protons would be consumed while exactly 2 protons would migrate through the CEM.
If a carbonate solution (K2CO3− (aq)) is fed into the electrolyzer 200, the reaction occurs accordingly:
2H++CO32−→H2O+CO2* (which consumes 2H+)
CO2*+2e−+H+→HCOO−(which consumes 1H+)
2H+ is transferred over the CEM 230. The carbonate solution consumes 1 proton. In comparison to the bicarbonate, pure carbonate would cause net proton consumption.
If CO32−(aq) is consumed by first combining with 2H+(aq) to form CO2*, the generation of each formate molecule would result in the net production of 1 OH− in the catholyte, thus leading to a carbon efficiency loss of 33%. The continuous increasing of the concentrations of CO32−/OH− triggers a vicious cycle and the FE and/or selectivity would decrease, ultimately leading to exclusive HER. Following the net reaction of KHCO3→HCOOK+½ O2, avoiding net acid or base accumulation that ultimately leads to HER. In order to maintain a high concentration of HCO3−(aq) while suppressing CO32− (aq) concentration, the maintenance of operational pH of a specific region to avoid the CO32−(aq) deep trap state is useful. Accordingly, a specific initial cathode electrolyte pH and protons as charge carriers is notably useful.
As show by graph 300, in equilibrium with ambient air where the CO2 partial pressure is 400 ppm (about 0.4 mbar (+/−0.01 mbar)), the carbonate species is dominate: [CO32−(aq)]>>[HCO3−(aq)]. Accordingly, a bicarbonate solution thermodynamically exudes CO2(gas) when in contact with normal air, resulting in a mixture of HCO3−(aq)/CO32−(aq). The mixture suffers from the carbon-efficiency problem and becomes almost inert for CO2ER at some point, because CO32−(aq) is directly inert toward CO2ER and will consume 2H+(aq) to boost its reactivity.
As shown by graph 400, when the partial pressure of CO2 in the overhead gas, PCO2, reaches 1 atm, the bicarbonate dominates in the aqueous phase: [HCO3−(aq)]>>[CO32−(aq)]. Accordingly, the bicarbonate solution may be converted into formate with a theoretical yield of about 100% (−4%) with nearly zero carbon efficiency loss, if in a constant CO2 partial pressure environment of PCO2=1 atm or higher. No continuous external CO2 gas source is called for, instead enough CO2(gas) to initially fill up the container headspace is enough. The amount of CO2(gas) is determined by the volume of the container headspace, accordingly the larger the container headspace the more CO2(gas). The pressure in the container headspace is 1 atm.
A key insight is the maintenance of steady-state H+(aq) concentration, which excludes the use of a single AEM in bicarbonate electrolyzers. Instead, the disclosed electrochemical cell in
In a first test, pH and pressure control are used to test the full cell voltage as a function of time. As demonstrated by line 630, almost pure H2 production can occur in the cathode chamber. Region 640 illustrates the depletion of KHCO3, with a formate yield of 94.60%.
Instead of using highly concentrated (e.g., greater than 1 M) bicarbonate solution,
Consequently, the aqueous CO2 solution is nearly completely converted into a fuel, with almost 100% carbon efficiency, at ambient pressure and temperature. Further, CO2 capture can be seamlessly integrated with CO2 electroreduction through the bicarbonate intermediate (in either highly concentrated liquid form, or 2-phase saturated liquid and solid-precipitate form after reaching the solubility limit), without going through the energy-consuming CaCO3 baking and gas compression.
In a second test, a 0.1 M KHCO3 solution at a pH of 9 (slightly higher initial pH than the solution in the first test illustrated by
Region 740 shows the results of the XRD patterns of solid products crystallized from the cathode of a full cell with 0.1 M KHCO3 at 20 mA cm−2, but without pH and pressure control. First line 742 illustrates the XRD pattern without pH and pressure control. For comparison, second line 744 is the XRD pattern of standard material K2CO3. As demonstrated by the first line 742, without pH, partial pressure control, and the disclosed electrolyzer the tested formate is impure and includes a plurality of K2CO3.
Enrichment of K+(aq) in the vicinity of the catalyst surface can lead to enhanced CO2ER selectivity due to the cation charge effect at the outer Helmholtz layer and increased concentration of the bicarbonate reactant. A higher concentration of bicarbonate solution can sustain a larger current density of 50 mA cm−2, until the near complete depletion of inorganic carbon solution. The high concentration bicarbonate solution was produced by increasing both the cation and anion concentrations. The resulting a concentrated (1.0 M) KHCO3 solution was electrolyzed, which produced a high formate conversion, as illustrated in region 840.
In comparison, a test can be conducted which fixed the HCO3-(aq) anion concentration to be 0.1 M while varying the K+(aq) cation concentration, with charge balanced by adding Cl−(aq) anion. There may be no significant differences in neutral-pH-solution electrolysis. In a further test, electrolysis of sodium bicarbonate solution in the same conditions can lead to slightly higher overpotential and lower FE.
In a further test, with a more concentrated (3.0 M) KHCO3 solution in a disclosed electrolyzer (e.g., that of
Higher current densities can lead to accelerated CO2 exsolution and the generation of carbonate. This may be addressed by incorporating a CO2/H2 separation membrane (e.g., Pd membrane) to in situ remove H2, or applying a higher-pressure reaction vessel to enable a higher concentration of dissolved CO2, or both. Elevated temperature results in CO2(g) exsolution, while high partial pressure results in CO2(g) dissolution. High temperature should also expedite reaction kinetics due to the Arrhenius law if the CO2(aq) is controlled as the same.
Described herein is a carbon-efficient bicarbonate electroyzer to obtain high-purity metal formate salts. The soluble metal formate, including potassium formate, sodium formate, and calcium formate, may be used in direct formate fuel cells (DFFC). In order to power DFCC, such solid salts only need to be dropped into water. DFFC possess a high volumetric energy density (e.g., about 53 g H2 per liter) and high specific energy density (e.g., about 2.13 kWh kg-1, which is more than 5 times that of conventional lithium-ion batteries).
Conventional DFFC may be operated with a power density of up to 591 mW cm−2 when paired with H2O2, or 302 mW cm−2 when paired with O2 gas, which is competitive against the standard H2 proton-exchange membrane (PEM) fuel cell (which produces a power density of about 700 mW cm−2 (+/−10 mW cm−2). Thus, opening the door for seasonal or even multi-year storage of the intermittent wind and solar electricity. Conventional cathode catalysts for DFFC include palladium black, which is a scarce and expensive metal.
Through active learning, the composition space of high-entropy metal nanoparticles may be explored, to enormously reduce the cost of the catalyst, while improving the performance. When tested with an in-situ electrodeposition method, the results of the disclosed salts demonstrate comparable or slightly better performance than the conventional Pd black catalysts, while using 25% of the palladium loading. Further, the incorporation of metals (including platinum and copper, etc.) is promising for furthering enhancing the performance, while still using low palladium loading. In this in situ electrodeposition method, one only needs to drop a metal chloride precursor solution onto the carbon strip, immerse the carbon strip into the reactor, and begin the reaction at a reducing potential. Densely and uniformly packed nanoparticles of about 40-50 nm in size then form in situ, said nanoparticles demonstrated a promising catalysis performance. Thus, this in situ electrodeposition method is highly facile and suitable for high-throughput production, which is an essential pre-requisite for applying active learning. On the other hand, in order to utilize the traditional method, one has to manually air-spray for an extended period of time to obtain a single sample, which is challenging to reproduce and scale up.
Reinforcement-learning-driven design may be used to form smart reactors for the carbon-efficient bicarbonate electrolyzers, such as the electrolyzer disclosed in
As discussed previously, a high CO2 partial pressure of above 1 atm is notably useful in achieving improved carbon efficiency. Accordingly, high-pressure flue gas may be directly upgraded, meaning the flue gas may be injected into the headspace of the bicarbonate reactor. The use of flue gas is desirable and less energy intensive than pure CO2 gas (which is produced from CaCO3 baking) and ensures the high CO2 partial pressure is provided.
The temperature management is important as well. As discussed, there are trade-offs in terms of temperature effects. An optimized temperature may be determined for the reactor. When the temperature is high, the reaction rate increases, which reduces the performance of the electrolyzer. Further, a high temperature results in additional reductions in electrolyzer performance, such as water evaporation and CO2 release. Optimized temperature may be determined through trial-and-error experiments.
A time-dependent voltage protocol may be used. A voltage that provides a reducing effect or an oxidizing effect may be useful to remove poisoning species on the electrode. The voltage may be determined by the properties of the material to be reduced or oxidized. Often, if the potential is higher than the redox potential then the voltage is oxidizing, if it is not then it is reducing. Poisoning species refers to molecules adsorbed on the electrode surface, which deactivate the surface or compete with the reactants for reaction. A time-dependent voltage is promising for periodically refreshing the active sites of the electrode and may lead to enhanced performance.
Accordingly, taking these intensive variables together, RL may be applied to optimize the operating conditions of a bicarbonate electrolyzer. In each RL step, the intensive variables are varied based on a certain step and the performance data is acquired (e.g., the Faradaic efficiency or carbon efficiency). Once the data is acquired for that RL step, the data is fed into the RL algorithm. The RL algorithm then analyzes the results in situ and informs the decision of the next step.
Disclosed herein is a carbon-neutral complete electrochemical conversion from bicarbonate to solid formate fuel. The electrochemical cell is configured to have a high concentration of HCO3−(aq) that is maintained while CO32−(aq) is maximally excluded in the cathode through control of at least one variable, the at least one variable including the pH of both the cathode and anode, a CO2(gas) partial pressure in the cathode chamber, and temperature. The high carbon efficiency, or a high yield of the conversion of bicarbonate to formate, is achieved because of an excess of HCO3-(aq) in the cathode and a lack of CO32−(aq).
Accordingly, the disclosed conversion process fits well into the global framework of carbon capture, utilization, and sequestration (CCUS). Various alkali basalt mineral rocks, containing CaO, MgO, SiO2, Na2O and K2O, are found in abundance on Earth and often used for enhanced weathering. Said mineral rocks are efficient for removing CO2 from the atmosphere and would form carbonates and bicarbonates. The insoluble components can be buried for carbon sequestration, while the soluble components (e.g., containing potassium, sodium and calcium, etc.), can be converted electrochemically with the disclosed electrolyzer and method into energy-rich solid formate fuels (see, for example,
The combination of high-efficiency bicarbonate-to-formate electrolyzer and DFFC may enable a “solid formate” economy, wherein volcanic alkali basalt rocks containing about 10 wt % (+/−1 wt %) K2O/Na2O/CaO are mined, and the soluble components used through the disclosed bicarbonate-formate cycle for long-term fuel generation and storage, and the insoluble left-over components (MgO/SiO2), after enhanced weathering in air, are used for geological sequestration. As all that is called for is H2 (g), and the disclosed bicarbonate-to-formate electrolyzer also produces nearly pure H2 (g) with only a trace amount of CO(gas) detected, the excess H2 (g) produced (with a 1−84.77%=15.23% Faradaic inefficiency) can be put to use by producing methanol liquid fuel at a local plant without long-range H2 transportation, thus providing a CO2-free conduit.
From captured liquid bicarbonate or saturated liquid/solid bicarbonate solutions, the electrolyzer and methods disclosed herein yield a formate of high purity with nearly 100% carbon efficiency and a notable current density at ambient pressure and temperature. The full-cell reaction achieved is 2KHCO3→2KOOCH+O2 with no net acid or base produced, thus the pH of both the catholyte and anolyte compartments are maintained with the use of a single CEM and buffer layer. The dominance of a high concentration of bicarbonate over carbonate by maintaining appreciable H+(aq) is of notable importance to achieving enhanced carbon efficiency and yield of CO2ER, and this can be implemented by the inclusion of a near-neutral-pH anolyte, a cation exchange membrane, and CO2 partial pressure management. The direct and complete conversion of highly concentrated bicarbonate is significant to the seamless coupling of carbon dioxide capture and electrochemical conversion.
Subject matter described herein (e.g., reinforcement-learning driven design of smart reactors and/or active-learning-drive design of catalysts) can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structural means disclosed herein and structural equivalents thereof, or in combinations of them. In some cases, the subject matter can be implemented as one or more computer program products, such as one or more computer programs tangibly embodied in an information carrier (e.g., in a machine-readable storage device), or embodied in a propagated signal, for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers). A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or another unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file. A program can be stored in a portion of a file that holds other programs or data, in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.
Processing described in this disclosure can be performed by one or more programmable processors executing one or more computer programs to perform functions of the subject matter described herein by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus of the subject matter described herein can be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).
Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processor of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of nonvolatile memory, including by ways of example semiconductor memory devices, such as EPROM, EEPROM, flash memory device, or magnetic disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.
A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.
Although reference is made herein to particular materials, it is appreciated that other materials having similar functional and/or structural properties may be substituted where appropriate, and that a person having ordinary skill in the art would understand how to select such materials and incorporate them into embodiments of the concepts, techniques, and structures set forth herein without deviating from the scope of those teachings.
It is to be understood that the disclosed subject matter is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The disclosed subject matter is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the disclosed subject matter. Therefore, the claims should be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the disclosed subject matter.
Although the disclosed subject matter has been described and illustrated in the foregoing embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the disclosed subject matter may be made without departing from the spirit and scope of the disclosed subject matter.
Claims
1. An electrolyzer, comprising:
- a cathode configured to receive a bicarbonate (HCO3−) solution;
- an anode configured to receive an anolyte; and
- a cation exchange membrane disposed between the cathode and the anode,
- wherein the electrolyzer is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
2. The electrolyzer of claim 1, wherein the bicarbonate (HCO3−) solution comprises potassium bicarbonate, sodium bicarbonate, or calcium bicarbonate.
3. The electrolyzer of claim 1, wherein the formate (OOCH−) solution comprises potassium formate, sodium formate, or calcium formate.
4. The electrolyzer of claim 1, wherein the bicarbonate (HCO3−) solution is a liquid and the formate (OOCH−) solution is a liquid or a mixture of a solid and a liquid.
5. The electrolyzer of claim 1, wherein the bicarbonate (HCO3−) solution is provided within a cathode chamber of the cathode, wherein the cathode chamber has a pressurized gas phase in a headspace of the cathode chamber sealed from an external air, wherein a pressure in the cathode chamber is controlled by a CO2 gas cylinder to between 0.01 MPa and 30 MPa.
6. The electrolyzer of claim 5, wherein the cathode chamber further comprises a CO2 gas in the headspace of the cathode chamber, wherein the CO2 gas cylinder controls a partial pressure of the CO2 gas to between 0.001 MPa and 30 MPa.
7. The electrolyzer of claim 5, wherein a temperature in the cathode is controlled using at least a heating pad to between 0 degrees Celsius and 300 degrees Celsius.
8. The electrolyzer of claim 1, wherein the anolyte has a specific initial pH between 5 and 7 and the bicarbonate (HCO3−) solution has a specific initial pH between 6 and 9.
9. The electrolyzer of claim 1, wherein the cathode includes a substrate loaded with tin nanoparticles.
10. The electrolyzer of claim 1, further comprising a buffer layer disposed between the cathode and the cation exchange membrane, wherein the buffer layer comprises one or more glass fibers impregnated with a bicarbonate.
11. A electrolyzer assembly, comprising:
- a cathode disposed in a cathode chamber having ports for receiving a bicarbonate (HCO3−) solution;
- an anode disposed in an anode chamber having ports for receiving an anolyte;
- a cation exchange membrane disposed between the cathode chamber and the anode chamber; and
- a buffer layer disposed between the cathode chamber and the cation exchange membrane,
- wherein the electrolyzer assembly is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
12. The electrolyzer assembly of claim 11, wherein the bicarbonate (HCO3−) solution comprises potassium bicarbonate, sodium bicarbonate, or calcium bicarbonate, wherein the bicarbonate (HCO3−) solution is a liquid, wherein the formate (OOCH−) solution comprises potassium formate, sodium formate, or calcium formate, and wherein the formate (OOCH−) solution is a liquid or a mixture of a solid and a liquid.
13. The electrolyzer assembly of claim 11, wherein the cathode chamber has a pressurized gas phase in a headspace of the cathode chamber sealed from an external air, wherein a pressure in the cathode chamber is controlled by a CO2 gas cylinder to between 0.01 MPa and 30 MPa.
14. The electrolyzer assembly of claim 13, wherein the cathode chamber further comprises a CO2 gas in the headspace of the cathode chamber, wherein the CO2 gas cylinder controls a partial pressure of the CO2 gas to between 0.001 MPa and 30 MPa.
15. The electrolyzer assembly of claim 11, wherein a temperature in the cathode is controlled using at least a heating pad to between 0 degrees Celsius and 300 degrees Celsius.
16. The electrolyzer assembly of claim 11, wherein the anolyte has a specific initial pH between 5 and 7 and the bicarbonate (HCO3−) solution has a specific initial pH between 6 and 9.
17. A electrolyzer assembly, comprising:
- a cathode disposed in a cathode chamber having ports for receiving a bicarbonate (HCO3−) solution, wherein the cathode chamber has a pressurized gas phase in a headspace of the cathode chamber sealed from an external air with a pressure in the cathode chamber controlled by a CO2 gas cylinder to between 0.01 MPa and 30 MPa, wherein a temperature in the cathode chamber is controlled using at least a heating pad to between 0 degrees Celsius and 300 degrees Celsius;
- an anode disposed in an anode chamber having ports for receiving an anolyte;
- a cation exchange membrane disposed between the cathode chamber and the anode chamber; and
- a buffer layer disposed between the cathode chamber and the cation exchange membrane,
- wherein the electrolyzer assembly is configured to convert the bicarbonate (HCO3−) solution to a formate (OOCH−) solution by movement of cations from the anode to the cathode via the cation exchange membrane.
18. The electrolyzer assembly of claim 17, wherein the bicarbonate (HCO3−) solution comprises potassium bicarbonate, sodium bicarbonate, or calcium bicarbonate, wherein the bicarbonate (HCO3−) solution is a liquid, wherein the formate (OOCH−) solution comprises potassium formate, sodium formate, or calcium formate, and wherein the formate (OOCH−) solution is a liquid or a mixture of a solid and a liquid.
19. The electrolyzer assembly of claim 17, wherein the cathode chamber further comprises a CO2 gas in the headspace of the cathode chamber, wherein the CO2 gas cylinder controls a partial pressure of the CO2 gas to between 0.001 MPa and 30 MPa.
20. The electrolyzer assembly of claim 17, wherein the anolyte has a specific initial pH between 5 and 7 and the bicarbonate (HCO3−) solution has a specific initial pH between 6 and 9.
Type: Application
Filed: Apr 1, 2024
Publication Date: Oct 3, 2024
Applicant: Massachusetts Institute of Technology (Cambridge, MA)
Inventors: Ju Li (Weston, MA), Zhen Zhang (Cambridge, MA), Zhichu Ren (Cambridge, MA)
Application Number: 18/623,432