Patent Application
20250235818
Carbon dioxide (CO2) capture from sources, such as industrial waste or the atmosphere, containing diffuse concentrations of CO2 is of interest for reducing CO2 emissions and for concentrating and/or purifying CO2 for further use or storage. Efforts and noticeable advancements were made in CO2 reduction reaction (CO2RR) Current thermal cycling carbon capture methods can have the disadvantage of operating at high temperatures and consuming fuel such as natural gas. Electrochemical processes that can operate at lower temperatures than thermal cycling and do not consume fuel, have been the subject of ongoing investigation. Furthermore, as the facilitation and distribution of renewable electricity becomes the viable alternative to the fossil fuels, electrochemically converting carbon dioxide back into basic chemical feedstocks has been perceived by numerous researchers as a promising method of storing and utilizing this renewable electricity while mitigating climate change. However, challenges remain.
This disclosure describes new processes and systems that are designed to continuously capture CO2 using electrolysis. In one or more embodiments, the process can be used to capture CO2 from different sources.
In one aspect, embodiments disclosed herein related to a device for carbon capture from a CO2 containing source. The device includes a cathode compartment including a cathode electrode for one or more reduction reactions, an anode compartment including an anode electrode for one or more oxidation reactions, a middle compartment which includes an ion conducting layer, a cation exchange membrane, and an anion exchange membrane. The middle compartment is separated from the cathode and the anode by the anion exchange membrane and the cation exchange membrane.
In another aspect, embodiments disclosed herein related to a system for carbon capture from a CO2 containing source. The system includes the device and a liquid/gas separator fluidly connected to the device and configured to separate an exit product obtained from the device into CO2-containing gas and a liquid.
In another aspect, embodiments disclosed herein relate to a method for carbon capture from a CO2 containing source. The method includes providing the device, supplying the CO2 containing source to the cathode of the device, reacting the CO2 from the source with an electrochemically generated species from the cathode to form a carbon species or directly reducing the CO2 at the cathode to form the carbon species, driving the carbon species to the middle compartment of the device, and reacting, in the middle compartment, the carbon species with an oxidation product from the anode of the device to form an exit product comprising CO2.
In general, one or more embodiments of the present disclosure relate to a method for carbon capture from a CO2 containing source using a device. The present disclosure allows the CO2 to be recovered from the CO2 containing source, which can then be stored and reused. The CO2 containing source may be of a source containing a low amount, or dilute amount, of CO2. Carbon loss generally introduces significant monetary and environmental strain in operation of CO2 electrolyzers, application of these devices would be viable for any type of future electrolyzer installations and operations. In the present disclosure, “carbon capture” refers to a process in which CO2 is removed from the CO2 containing source, or a process in which CO2 is recovered from the CO2 containing source, or a combination of the CO2 removal and recovery processes.
One or more embodiments relate to devices, systems and processes that are designed to capture CO2 gas from the cathode during electroreduction. Systems and devices of one or more embodiments may include a cathode, anode, and a middle compartment separating the cathode and anode. This technology utilizes the carbon species ionic transfer phenomenon but avoids direct interaction with the anode side by introducing a middle compartment between the two electrodes to isolate and recover these anions as pure CO2 or carbon species (such as carbonate or bicarbonate) feedstocks.
The process, device and the system of the present disclosure use one or more reduction reactions at the cathode. Suitable reduction reaction reactions for effective CO2 capture include CO2 reduction reaction (CO2RR) and oxygen reduction reaction (ORR). In CO2RR, OH− ions are generated as a result of CO2 reacting with water. OH− ions then reacts with CO2 to produce carbon species such as carbonate and bicarbonate ions. In ORR, OH− ions are generated as a result of O2 reacting with water. The ORR may include reacting O2 via the classic ORR to create a strong interfacial alkaline environment for CO2 capture. OH− ions then reacts with CO2 to produce carbon species in a similar manner as the case for CO2RR. There are no specific chemical inputs (other than water) or consumption during the capture process since the device performs redox electrolysis such as CO2RR/OER or ORR/OER. In addition, due to the triple-phase boundary created at the cathode of the device, CO2 can diffuse rapidly in the gas phase towards the catalyst/membrane interface and be simultaneously absorbed by the interfacial OH− ions, which may enable the reactor to be operated under large current densities for rapid CO2 capture while still maintaining high Faradaic efficiencies (carbonate species instead of hydroxide crossover).
The devices, systems and processes of the present disclosure show continuous and stable operation for 70 hours under 100 mA/cm2. The current density and stability of these devices can be improved substantially by optimizing the catalyst selection, the flow parameters, the electrolyte design, and the device design such as, for example, a thinner solid electrolyte layer.
The strategy provided in one or more embodiments is to provide a “buffer layer” between cathode and anode that could neutralize formed carbonate ions to regenerate CO2 gas before they reach to the anode side.
The CO2 containing source may include O2, CO, H2, NOx, SOx, H2O, Ar, CH4, and N2, where x is a non-zero integer. As used herein “include(s)“means” include(s) but is not limited to.” The CO2 containing source may contain CO2 gas in an amount of 10 ppm or more. In one or more embodiments, the CO2 containing source is diluted such that the source contains CO2 gas in an amount of 400 ppm to 50%.
The method for carbon capture from a CO2 containing source includes providing a device. The device may include a cathode compartment including a cathode electrode for one or more reduction reactions, an anode compartment including an anode electrode for one or more oxidation reactions, a middle compartment which comprises an ion conducting layer, a cation exchange membrane, and an anion exchange membrane. The middle compartment is separated from the cathode and the anode by the anion exchange membrane and the cation exchange membrane.
The method includes supplying the CO2 containing source to the cathode, reacting the CO2 from the source with an electrochemically generated species from the cathode to form a carbon species, or directly reducing the CO2 at the cathode to form the carbon species, driving the carbon species to the middle compartment; reacting, in the middle compartment, the carbon species with an oxidation product from the anode to form an exit product including CO2.
The systems and processes of one or more embodiments employ solid electrolyte reactors (a “device,” a “solid electrolyte derive”) to capture CO2 during the electrolysis reactions. The device includes ion-conducting polymers between the anode and the cathode of the reactor. The electric field generated during the operation drives the cathode-side generated carbon species ions. To travel across the anion exchange membrane into the middle compartment, and this system uses this middle compartment as a recombination site or reacidification site to regenerate CO2 gas in the middle compartment.
The systems and devices of one or more embodiments serve to mitigate the carbon loss due to interfacial alkalinity-dependent carbonate (CO32−) or bicarbonate (HCO3) formation by isolating these anions in a buffer layer located between cathode and the anode of the electrochemical cell. Embodiments of the present disclosure further provide systems and processes which electrochemically separate the CO2 gas from input gas mixture utilizing the carbonate/bicarbonate cross-over and isolation in the buffer layer.
The exit product, which contains CO2 and water, is introduced to a liquid/gas separator 210 to separate CO2 gas from the liquid. CO2 gas may exit the liquid/gas separator 210 and temporarily stored in a vessel 220. The recovered CO2 may be recirculated back to be combined with the CO2 containing source to be introduced to the device 100, or may be collected to be used for various purposes. The liquid obtained from the liquid/gas separator 210 may be recirculated to the middle compartment 120 to be used to remove CO2 from the middle compartment 120.
The exit product, which contains CO2 and water, is introduced to a liquid/gas separator 210 to separate CO2 gas from the liquid. CO2 gas exit the liquid/gas separator 210, and collected as recovered CO2. The liquid obtained from the liquid/gas separator 210 may be recirculated to the middle compartment 120 to be used to remove CO2 from the middle compartment 120. The device 100 and the systems 200 and 300 of one or more embodiments allow carbon capture without the use of any chemicals.
Various features of the device 100 and the systems 200 and 300 may be combined as required for specification applications. For example, the system 200 may include the anolyte tank 310 and the anolyte may be recirculated between the anolyte tank 310 and the anode compartment 116, and the generated O2 in the anode compartment 116 may be combined with the CO2-containing source.
In one or more embodiments of the present disclosure, the device 100 and systems 200 and 300 are an electrochemical device performing electrolysis to separate CO2 from the cathode gas stream.
In one or more embodiments, the system and device are operated with CO2-containing cathode inlet gas flow. The concentration of CO2 and mixture composition is not limited. Generally, one or more embodiments of the present disclosure could deal with as high as pure CO2 inlet flow used for CO2 reduction reaction (CO2RR) and a dilute CO2 inlet flow, such as CO2 inlet flow of as low as 10 ppm CO2 concentration. The CO2 inlet flow may be coupled with cathodic reactions involving the generation of CO2 reactive species such as OH−, HO2−, or other molecules or nucleophiles that bind CO2.
In one or more embodiments of the present disclosure, the cathode compartment 110 includes a cathode electrode 112 for one or more reduction reactions. Any aqueous redox couple, reduction in cathode and oxidation in anode, can be used with the device and the system. Reduction reactions in the cathode compartment 110 may include an hydrogen evaluation reaction (HER), oxygen reduction reaction (ORR), oxygen reduction, CO2 reduction reaction (CO2RR), carbon monoxide (CO) reduction reaction (CORR), nitrogen (N2) reduction reaction (NRR), a nitrate reduction reaction (NO3RR), a nitrite reduction reaction, a reduction reaction producing a species that absorbs CO2, and combinations thereof. Additionally, the reduction reactions in the cathode compartment 110 may include an oxygen reduction reaction to form an electrochemically generated hydroxide ion species. The cathode compartment 110 may further include a catalyst.
Examples of the cathode electrode 112 may include a gas diffusion layer (GDL). Examples of the catalyst comprised in the cathode compartment may include platinum on carbon catalyst (Pt/C), silver nanowire catalyst (Ag NW), 2-dimensional bismuth nanosheet catalyst (2D-Bi), copper nanoparticles (CuNP), oxidized carbon black (OCB), and transition metal single-atom catalysts (TM-SAC) such as nickel single atom catalyst (Ni-SAC), iron single atom catalyst (Fe-SAC), and cobalt single atom catalyst (Co-SAC). The operation of the device and system is independent of target CO2 reduction products, both gas and liquid.
The catalyst may be contained on carbon, such as N-doped carbon. The catalyst may be disposed on the surface of the cathode electrode via a process such as spray coating.
In one or more embodiments of the present disclosure, the anode compartment 116 includes an anode electrode 118 for one or more oxidation reactions. The oxidation reactions may include oxygen evolution reaction (OER) to form oxygen and protons. The oxidation reaction may include a hydrogen oxidation reaction (HOR) to form protons. The anode compartment 116 may further include a catalyst. Examples of the anode electrode 118 may include gas diffusion layer (GDL) and IrO2.
Various anolytes can be used depending on what oxidation reaction is being targeted on the anode side, as well as how the system aims to recover carbonate/bicarbonate ions. Many conductive ion-containing liquid electrolytes can be used as the anode provided that the anolytes are suitable for the oxidation reaction. For example, with OER, sulfuric acid (H2SO4), water, sodium bicarbonate (NaHCO3), potassium hydroxide (KOH) can be used. Using acid, water and other electrolyte without cations can be used if the goal is to regenerate isolated carbonate/bicarbonate ions as CO2 gas. The protons will mobilize during the oxidation reaction across the CEM 126 into the buffer layer in the middle compartment 208 to protonate the carbonate/bicarbonate ions. In embodiments using cation-containing electrolytes such as KOH, the recovery of isolated carbonate/bicarbonate ions as liquid may be accomplished. The cations instead of protons would travel across the CEM 126 to combine with the carbonate/bicarbonate ions to generate liquid solution products.
The middle compartment 120 may include an ion conducting layer, or a “buffer layer,” and may be separated from the cathode and the anode by an AEM 124 and a CEM 126. In one or more embodiments the ion conducting layer includes a porous solid electrolyte or a liquid electrolyte. The buffer layer can be in any physical states such as liquid, solid, or gas.
The buffer layer may have high ionic conductivity that can ensure a small ohmic drop between cathode and anode for high energy efficiencies. The buffer layer may have a pH in a range of about 3 to 11, or at around neutral range (such as CO2 saturated water with a pH of about 5), as high alkaline pH substantially increases the CO2 crossover rate due to carbonate formation, and low acidic pH may damage the AEM and lower the catalytic performances. The buffer layer may have properties such that the buffer layer would not be consumed continuously by reacting with crossover CO2, as such buffer layer would make the system unsustainable.
One or more embodiments include a porous solid electrolyte layer as the buffer layer. The solid electrolyte layer may contain dense but permeable ion-conducting polymers functionalized with sulfonate groups, which guarantees efficient proton conductions between cathode and anode for very small ohmic drops. The cathode side reduction reaction results in the formation of hydroxide ions which react with free CO2 gas to form a carbon species, such as carbonate ions. The carbon species driven by the electrical field then migrate across the AEM into the solid electrolyte layer, where they are recombined with protons generated from the anodic OER to compensate for the charge. Therefore, the porous solid electrolyte layer serves as a recombination site for the carbon species and protons, while not sacrificing carbon capture performances as demonstrated in our previous solid electrolyte reactor systems.
The protonation of carbonate results in the formation of dissolved CO2 and CO2 gas, which are completely separated from the anode O2 stream. By recycling a DI water stream saturated by CO2 through the porous solid electrolyte layer to remove regenerated CO2 gas, a continuous recovery of high purity CO2 gas that can be readily reused in the reduction reaction at the cathode can be obtained. One or more embodiments employ sulfonate group containing polymers as the porous solid electrolyte in the middle compartment. However, other variations of ion conducting polymers or resins may be used in the middle compartment provided that they do not damage the other components of the device, such as the membranes. The porous solid electrolyte layer may include a Nafion membrane. As noted above, one or more embodiments include a buffer layer to isolate carbon species such as carbonate/bicarbonate where the buffer layer can be non-restrictive if it meets certain criteria. The buffer layer placed between the two electrodes but is separated from the cathode by AEM 124 and anode by CEM 126 is capable of transporting ions. Electrochemical cells generally require facile ion transportation between cathode and the anode, which includes middle compartment in the present device and system.
In one or more embodiments, the buffer layer in the middle compartment contains a small amount of cations. Such addition of cations in the buffer layer may improve the stability and capture performance of the device.
The AEM 124 may be any material that allows the carbon species, such as carbonate and bicarbonate ions, to transport across the membrane while preventing molecules such as CO, CH4 and C2H4 from transported across the membrane and reaching to the middle component 208. Specific examples of AEM 124 includes Dioxide Material X-35.
CEM 126 may be any material that allows the oxidation product, such as proton ions, to transport across the membrane while preventing molecules such as O2 from transported across the membrane and reaching to the middle component 120. Specific examples of CEM 126 includes sulfonated tetrafluoroethylene such as Nafion™ 115 proton exchange membrane (PEM).
Method for Carbon Capture from a CO2 Containing Source
In one or more embodiments, the method for carbon capture from a CO2 containing source includes providing the device as previously described. In one or more embodiments, one device is provided to conduct the carbon capture. In one or more embodiments, a plurality of the device is provided to conduct the carbon capture. The plurality of the device may be connected in series, parallel or combinations thereof.
The method may include supplying a CO2 containing source to the cathode of the device. At the cathode, the CO2 gas may react with an electronically generated species from the cathode to form a carbon species. Alternatively, the CO2 gas may be directly reduced at the cathode to form a carbon species.
The carbon species may be ionic or non-ionic. When the carbon species is ionic, the ionic carbon species may include, but are not limited to, carbonate, bicarbonate, CO2-molecule complex, percarbonate and combinations thereof. In case of carbonate ion crossover, recombination of carbonate and generated protons (from the anode) allows the formation of CO2 which then can be recovered as ultra-high purity CO2 gas.
In one or more embodiments, the method includes driving the carbon species to the middle compartment 120. The carbon species may be driven to the middle compartment 120 either electrically or by mass-diffusion.
In one or more embodiments, the method includes reacting, in the middle compartment 120, the carbon species with an oxidation product from the anode to form an exit product comprising CO2. The exit product may have a higher concentration of CO2 gas relative to the source. The exit product may include CO2 and water. The water included in the exit product may be formed as a result of the reaction between the carbon species and the oxidation product.
In one or more embodiments, the method further includes supplying an oxidation input to the anode compartment 116. The oxidation input includes one or more of protons, H2, H2O, NH3, HCOOH, CO, methanol, ethanol, acetic acid, and an oxidation producing species to regenerate CO2 in the middle compartment. The oxidation input may be contained in the anolyte supplied to the anode compartment 116.
In one or more embodiments, the method includes combining CO2 included in the exit product with the CO2 containing source. Addition of recovered CO2 to the CO2 containing source may saturate the CO2 containing source, allowing CO2 to be recovered in gaseous form in the middle compartment 120.
In one or more embodiments, the method includes generating O2 in the anode compartment and combining the generated O2 with the CO2 containing source. The generation of O2 may occur as a result of the oxidation reaction in the anode compartment 116 conducted to produce the oxidation input.
The method for carbon capture from a CO2 containing source may be conducted continuously or intermittently.
In one or more embodiments of the present disclosure, the carbon capture may include either of CO2 removal or CO2 recovery.
In one or more embodiments of the present disclosure, the carbonate Faradaic efficiency and the CO2 removal efficiency may be of 70% or more, or 90% or more.
In one or more embodiments of the present disclosure, the carbon capture rate may be 3 mLCO2 min−1 cm−2 or more under room temperature and ambient pressure.
Further mechanisms and example performances of this devices are detailed below.
A high-efficiency recovery of the crossover CO2 during CO2RR electrolysis using a porous solid electrolyte reactor design is demonstrated. CO2 may be recaptured by introducing a porous solid electrolyte buffer layer between cathode and anode, and combining crossover CO32− with protons from anode OER to form CO2 gas, and continuously flushing the porous solid electrolyte layer with deionized (DI) water.
A device shown in the schematic of
Studies were conducted to determine the accuracy of the measurement of CO2 crossover rate and CO2 recovery rate. First, on the cathode side of the device, the 20 standard cubic centimeters per minute (“sccm”) CO2 input stream were split into three components, converted CO2 (to CO), crossover CO2 (to solid electrolyte layer), and remaining downstream CO2. CO2 conversion rates and downstream CO2 flowrates were measured in order to obtain the CO2 crossover rates under different cell operation conditions.
CO2 conversion rates can be readily calculated based on the CO/H2 quantification using gas chromatography (GC) and the cell current. On the contrary, the measurement of downstream CO2 flowrates may be difficult due to the small flowrate changes compared to its baseline, especially under low cell currents.
One method of downstream CO2 flowrates is to directly connect the downstream to another mass flow controller (MFC) at the outlet and translate its flowrate readings back to CO2 flowrate by considering the generated CO and H2 gas components (“MFC method”).
A more sophisticated and reliable gas analysis system was also developed (“GC method”). 200 sccm of Ar as a carrier gas and 5 sccm of the internal integration standard gas were mixed with the downstream gas flow from the cathode of the device before fed into the GC. Ethylene (C2H4) was mostly used as the internal integration standard gas, while methane (CH4) was used for the systems where CO2RR products includes C2H4 such as the Cu catalyst (no CH4 is produced in the present case).
Flowrates of all supplied gas (CO2, Ar, C2H4 and CH4) were precisely controlled by MFC (Alicat Scientific) with different ranges. For example, 20-sccm inlet CO2 was controlled by a 50-sccm range MFC. The MFC has an accuracy of ±(0.8% of Reading+0.2% of Full Scale) based on the product specification. The accuracy tolerance of MFC translates to an error range of ±0.26 sccm when supplying a 20 sccm CO2 stream (˜1% error).
The downstream gas on the cathode side was mixed with 200 sccm of Ar carrier gas and 5 sccm of internal standard gas which is CH4 for copper testing and C2H4 for all other tests. This mixture was then fed to the GC (SRI 8010C) where flame ionization detector (FID) with methanizer was used for CO2 quantity analysis and thermal conductivity detector (TCD) was used for O2 quantity analysis.
The output signal contains CO2 and internal standard gas peak, and both peaks were integrated to determine their respective areas. The ratio between these two areas provide the actual downstream CO2 flowrate based on the GC calibration curves (
The CO2 crossover rate measured by the GC method and the MFC method are shown in
Dissolved CO2 and CO2 gas bubbles in the DI water flow stream were measured separately using titration and water displacement method, respectively, to determine the CO2 crossover rate into the solid electrolyte buffer layer. The crossover CO2 gas dissolved in DI water emerges as gas bubbles when water become saturated. In continuously circulating operations, all crossover CO2 would be collected in the gas phase once the circulating DI water stream becomes saturated.
The titration method was used to accurately detect the amount of dissolved crossover CO2 in all forms (carbonic acid, carbonate, bicarbonate, and dissolved CO2 equilibriums) within the DI water flowing through the middle compartment. The Middle compartment output stream containing dissolved CO2 was collected directly in 200-500 μl of 1M NaOH (volume of NaOH was changed to ensure the pH of the collected solution would be greater than 10). The middle compartment output stream was collected in alkaline solution to minimize the loss dissolved CO2 in order to provide accurate titration results. 5 ml of this liquid was titrated using 0.1M HCl and pH meter (Orion Star A111) to obtain the CO2 flow rate equivalent of dissolved carbon concentration.
In order to confirm the accuracy the amount of dissolved CO2 measured by the titration method, a validation study was conducted by preparing and titrating a K2CO3 standard solution. The K2CO3 standard solution was prepared to have a carbonate ion concentration similar to the water saturated with CO2 that is expected to be produced during the crossover CO2 recovery operation, which is about 33.5 mmol/L, or 0.9 sccm CO2 flowrate equivalent (dissolved in 1.1 mL/min Dii water flow). The titration was conducted using 0.13 M HCl solution and 5 ml of the standard solution. “Equivalence points” represent points at which the concentration of protonated ions in the solution is equal to the concentration of non-protonated ions. At the equivalence point near the pH of 8, the concentration of CO32− was equal to that of HCO31−. At the equivalence point near the pH of 4, the concentration of HCO32− was equal to that of H2CO3. The concentration of dissolved CO2 may be estimated by determining the difference between the two equivalence points.
CO32− concentration of the standard solution was prepared and obtained via titration. The measured CO32− concentrations were approximately 1 mM lower than expected. The difference corresponds to approximately 0.03 sccm of CO2 flow equivalent error.
CO2 saturated 0.05M H2SO4 was used as the solvent during the water displacement measurement to measure the CO2 bubble flowrate in the heterogenous middle layer downstream flow. Acid was used to minimize the gas dissolution during the bubble flowrate measuring process, as CO2 gas has substantially lower solubility in acidic solution than water. The acidic solution was also pre-saturated with CO2.
The accuracy of the water displacement method was validated by comparing the data to the CO2 flow controlled by MFC. The flowrates measured by water displacement method were very close to the standard value controlled by MFC (less than 1.3% standard error) and that the volumetric flow rate of CO2 gas can be reliably measured.
For CO2RR gas product analysis, the downstream gas was directly sent to the GC (Shimadzu GC 2014) without mixing with carrier gas or internal standard gas. The partial current of the measured gas product is calculated by following equation:
where xi is the volume fraction of the product determined by online GC referenced to calibration curves from the standard gas sample, v is the flow rate of 20 sccm, ni is the number of electrons involved, p°=101.3 kPa, F is the Faradaic constant, T=298 K and R is the gas constant. Faradaic efficiency, then, is calculated using the equation below:
The downstream gas flowrate on the cathode-side changes because of the CO2 gas conversion or crossover. Thus, the selectivity of H2 and CO in Ag NW and Ni-SAC obtained from the GC (calculated with the assumption that total flow rate of downstream gas is still 20 sccm) was later normalized to 100% after confirming the absence of other gas or liquid products. In the case of the CuNP, various CO2RR products were co-generated. The selectivity was tested with increased input CO2 flowrate to minimize the error arising from flowrate discrepancy in the GC instead of normalization.
One-dimensional 1H NMR spectra measured from Bruker AVIII 500 MHz NMR spectrometer was used to analyze the liquid product obtained in the experiment conducted using 2D-Bi and CuNP catalysts, as described below. For the NMR test, the 500 μl of the middle layer output liquid was mixed with 100 μl of D2O (Sigma-Aldrich, 99.9 at % D) and 0.05 μl dimethyl sulfoxide (Sigma-Aldrich, 99.9 at % D) as internal standard.
SEM image analyses were conducted using FEI Quanta 400 field-emission SEM. TEM characterizations for Ag NW, 2D-Bi, and CuNPs were carried out using FEI Titan Themis aberration-corrected TEM at 300 kV. Aberration-corrected MAADF-STEM images for Ni-SAC were captured in a Nion UltraSTEM U100 operated at 60 keV and equipped with a Gatan Enfina electron energy loss spectrometer at Oak Ridge National Laboratory.
Electrochemical measurements for EXAMPLE 1 were conducted using a Bio-Logic VMP3 workstation. The porous solid electrolyte reactor containing catalysts to be evaluated were loaded on 2.5 cm2 GDL as the cathode electrode. A polytetrafluoroethylene (PTFE) gasket with 2.5 cm−2 window and Dioxide Material X-35 was placed between the cathode electrode and the solid electrolyte layer as an AEM. A proton conducting polymer electrolyte, Dowex 50W X8 hydrogen form Sigma-Aldrich was used to pack the middle compartment. Nafion™ 115 film from Fuel Cell Store with another PTFE gasket was used as CEM to separate anode from the middle compartment. IrO2 was used as the anode for oxygen evolution reaction.
During the operation of the solid electrolyte reactor without DI water recycling system, the cathode was supplied with 20 sccm humidified CO2 for all tests. When the Faradaic efficiency (FE) for Cu sample was tested, the flowrate of inlet CO2 was temporarily increased to 50 sccm in order to minimize the FE measurement error associated with CO2 stream flowrate change. 1.1 ml/min of DI water was continuously introduced to the middle compartment to remove dissolved CO2 and CO2 gas, and the anode side was circulated with 2.7 ml/min of 0.5 M H2SO4.
For the solid electrolyte layer with recycled DI water stream, the measurement was conducted in the same manner except that the anolyte was replaced with 2.7 ml/min of DI.
The cell resistance was measured by the potentiostatic electrochemical impedance spectroscopy (PEIS). The cell voltages for the two electrode systems were manually adjusted with 60% iR compensation to avoid overcompensation. The typical impedance of the solid electrolyte reactor was measured to be approximately 2 to 3Ω including electrical connections to the instrument.
The CO2 recovery characterization study was conducted based on the method for carbon capture from a CO2 containing source using the device as shown in
Ag NW was used as the CO2-to-CO catalyst which was placed on the cathode of the device (porous solid electrolyte reactor) as described previously. Ag NW-L70, available from ACS Material Store, was used as purchased.
IrO2 was used as the anode electrode to oxidize water to O2 and to continuously supply protons to the solid electrolyte layer across Nafion™ cathode exchange membrane (proton exchange membrane (PEM). The Ag NW catalyst had a uniform diameter of approximately 70 nm. Analysis conducted by a high-resolution transmission electron microscopy (HRTEM) showed that Ag NW catalyst exhibited a lattice structure having a lattice spacing of 0.242 nm. The lattice structure also shows that the surface of the Ag NW is mainly covered by (111) facet, which was identified as the active surface for CO2RR to CO. Ag NW-containing cathode electrode was prepared by loading approximately 0.8 mg/cm2 of Ag NW obtained from ACS Material Store onto Sigracet 28BC GDL electrode (available from Fuel Cell store) with 5% Nafion 117 polymer binder solution available from Sigma-Aldrich.
The I-V curve of CO2RR in the solid electrolyte reactor with Ag NW catalyst is shown in
Results provided in
The gas purity of the recovered CO2 stream was above 99% as confirmed by GC measurements (See
The CO2 recovery performances of the porous solid electrolyte reactor with different CO2RR catalysts and products was performed. Ni single atom catalyst (Ni-SAC) was evaluated as it has been demonstrated to have high selectivity for CO. Microscopic image analysis showed that Ni-SAC exhibits porous morphology of the support carbon material and the Ni atoms are well dispersed in the carbon matrix. Ni-SAC was prepared as described in K. Jiang et al., Isolated Ni single atoms in graphene nanosheets for high-performance CO2 reduction, Energy Environ Sci., 11, 893-903 (2018). In preparation of Ni-SAC, well dispersed Ni atoms were placed onto graphene nanosheets.
It was hypothesized that the CO2 crossover rates may be dramatically different if different anion CO2RR products, such as formate, are produced because less numbers of OH− groups are generated. (See Table 1 and
CO2RR to formate is a two-electron transfer process (the same as CO), but only one OH− ion is produced and the other charge is compensated by HCOO− (See Table 1).
The CO2 recovery based on CO2RR to multi-carbon products was investigated by conducting the above-described experiment except that Cu nanoparticles (CuNP) were used as the catalyst for the cathode electrode. The cathode electrode was prepared as previously described. CuNP, available from Sigma-Aldrich, was used as purchased. The CuNP was shown to contain 30-50 nm diameter nanocrystals with uniform morphology, and while the Cu particles were agglomerated, the particles retain their nano structures.
In sum, above results demonstrate that over 90% of the crossover CO2 gas in an highly purity form (over 99% gas purity) can be consistently recovered, while maintaining CO2RR catalytic performance of over 90% CO selectivity and high current density of 200 mA/cm2. The results also demonstrate that the porous solid electrolyte reactor design for crossover CO2 recovery can be successfully extended to different CO2RR catalysts and products.
In practical carbon capture operations, a direct recovery of crossover CO2 in its gas phase, would be desirable compared to CO2 partially dissolved in water, because the captured CO2 can be directly fed back to the main CO2 stream. This can be achieved by continuously recycling the saturated DI water stream through the porous solid electrolyte layer as shown
Using the setup as shown in
CO2 crossover rates as measured by GC and the water displacement method indicate that the there is no reduction in CO2 recovery with respect to time, with over 80% of the crossover CO2 shown to be recovered. The cell voltage was stable (approximately 3.6 V) and high CO FE was maintained without substantial change in CO selectivity (approximately 90%) throughout the test.
Image analyses of the Ag NW catalyst before and after the continuous operation showed no visible change of the catalyst, indicating good structure stability of the catalyst.
The crossover CO2 remained relatively constant at approximately in a range of 1.4 to 1.5 sccm. The crossover rate was slightly lower compared to the crossover in which fresh DI water was continuously introduced into the reactor (as shown in
The gas recovery rate was also monitored during the long-term operation and it was maintained around 1.2 to 1.3 sccm range. For the entire duration, more than 80% of the crossover CO2 measured from the cathode-side was recovered in the middle compartment for the entire duration of the test, indicating consistent and efficient CO2 recovery by the device.
In EXAMPLE 2, additional embodiments of the method for carbon capture from a CO2 containing source is provided based on O2/H2O electrolysis coupled with the porous solid electrolyte (PSE) reactor.
The electrochemical measurements of EXAMPLE 2 were conducted by mixing 40 mg of as-prepared catalysts, 4 ml of 2-propanol (Sigma Aldrich) and 160 μl of Nafion binder solution (Sigma, 5%) to form a catalyst ink with approximate density of 10.0 mg mL−1. The ink was sonicated for about 30 minutes to obtain a homogeneous ink and then was spray coated onto the 5×5 cm2 Sigracet 28 BC gas diffusion layer (Fuel Cell Store) electrodes. The Pt/C (Fuel cell store) used in this work followed the same procedure to prepare the cathode electrode. The IrO2 electrode available from Dioxide Materials was used for the anode electrode.
The electrochemical measurements were conducted using a BioLogic VMP3 workstation. Respective catalysts (Pt/C, Co-SAC, for example), were loaded on 1.0 cm2 gas diffusion layer (GDL) as the cathode electrode. A 0.5 mm thick PTFE gasket with 1.0 cm−2 window and AEM membrane were placed between the cathode electrode and the solid electrolyte layer. The middle compartment included 2.5 mm Delrin plastic (1.5 mm for thinner middle layer plate) and was packed with Dowex 50W X8 hydrogen form solid electrolyte to ensure ionic conductivity. Nafion 117 film (Fuel Cell Store) with a second PTFE gasket was placed between the anode and the solid electrolyte layer, as ECM.
During the reduction process, the cathode was supplied with humidified CO2 and 200 standard cubic centimeter per minute (sccm) O2 mixtures for all tests, unless otherwise described.
For direct air capture tests, the input gas CO2 concentration was set to 400 ppm (available from Airgas) the total air gas flow was increased to 1000 sccm to ensure sufficient CO2. 6-cm2 electrode was used to increase the total carbon capture current for minimized measurement errors in carbon capture rates.
All gas flowrates (CO2, O2, N2 and air) were precisely controlled by the mass flow controller (Alicat) and the concentration of the mixture was measured and recorded by a CO2 meter. 1.1 ml/min (0.5 mL/min for DAC) of DI water was continuously introduced to the middle solid electrolyte layer to remove dissolved CO2 and CO2 gas, and the anode side was circulated with 2.0 ml/min of DI water or 0.1 M H2SO4 (>300 mA cm−2). For the long-term stability test, anolyte was replaced with 2.0 ml/min of DI water, while keeping the rest of the conditions the same. The cell resistance was measured by the potentiostatic electrochemical impedance spectroscopy (PEIS), and the cell voltage was reported without any IR compensation.
The DI water used to remove CO2 from the middle compartment was pre-saturated with Ar in order to avoid introducing any CO2 contamination from external sources into our PSE layer. As a result, a fraction of captured CO2 dissolved into our DI water stream which requires titration. As previously noted, DI water may be recycled in practical application to saturate the water with CO2 such that the captured CO2 would be removed from the middle compartment in gas form.
The middle layer output stream containing dissolved CO2 was collected directly in 200-500 μl of 1 M NaOH. The loss of dissolved CO2 to air was minimized by collecting in alkaline solution, and a full range of titration could be conducted. 4 ml of this collected liquid was titrated using 0.1 M HCl and pH meter (Orion Star A111). The volume difference between two equivalence points on the titration curve determines how many moles of carbonate species exist inside the liquid samples. The dissolved carbon dioxide concentration was then calculated as below:
Where Q1 is the CO2 flow rate equivalent to dissolved carbon concentration, ΔV is the volume of HCl between two equivalence points on the titration curve, C1 is the concentration of the HCl solution used, 24.4 (mol/L) is the molar volume of an ideal gas at 1 atmosphere of pressure, V is the volume of the sample titrated, and q is the flow rate of the collected liquid output.
The partial current density for a given gas product was calculated as below:
where Q1 and Q2 is the volumetric flow rate of liquid and gaseous CO2 determined by titration and water displacement method, n is the number of electrons involved, which is 2 for carbonate Faradaic efficiency, and F is the Faradaic constant.
SEM analyses were conducted using FEI Quanta 400 field-emission SEM. TEM characterizations and EDS elemental mapping images for SACs were carried out using FEI Titan Themis aberration-corrected TEM at 300 kV. XPS data was collected on a PHI Quantera spectrometer, using a monochromatic Al Kα radiation (1486.6 eV) and a low-energy flood gun as a neutralizer. All XPS spectra were calibrated by shifting the detected carbon C is peak to 284.6 eV. N2 adsorption-desorption isotherms were recorded on a Quantachrome Autosorb-iQ3-MP instrument at 77 K using Barrett-Emmett-Teller calculations for the surface area. X-ray absorption spectroscopy (XAS) measurement and data analysis. XAS measurements were performed at the soft X-ray Microcharacterization Beamline (SXRMB) of the Canadian Light Source (CLS). Metal foils and metal oxides were used as references. The acquired EXAFS data were extracted and processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages.
The CO2 recovery characterization study was conducted based on the device as described in EXAMPLE 1, except that the Ag NW catalyst was replaced with platinum on carbon (Pt/C) as the cathode catalyst for ORR. IrO2 was used as the anode for OER.
Validation of the titration and water displacement methods was conducted in a similar manner as previously described in EXAMPLE 1. Na2CO3 standard solution with carbonate concentration of 35 mM and 3.5 mM were prepared and titration was conducted using 0.1 M HCl and 0.02 HCl solutions and 4 ml standard solution.
The recovered gas product from the porous solid electrolyte (PSE) layer was analyzed by GC.
Based on the reaction mechanism as discussed above, the maximum current density to maintain high FE is expected to decrease with lower CO2 concentration in the input gas, as a result of limited mass diffusion. This is confirmed by the FEcarbonate test data based on 8.6% and 4.6% CO2 in the input gas, as shown in
The CO2 mass transport from the mainstream flow to the catalyst/membrane interface is the primary factor that restricts the carbon capture rate when the input gas has low CO2 concentrations. This limitation, particularly in direct air capture (DAC) applications in which the concentration of CO2 may be 400 ppm (0.04%), generally poses a challenge in various carbon capture processes.
The flowrate of the CO2-containing input gas was adjusted such that there is over 80% CO2 left over in the tail gas (less than 20% crossover) in order to avoid insufficient CO2 supply.
Similar to the experiments conducted in EXAMPLE 1, alternative ORR catalysts were investigated for their carbon capture performance. Replacing Pt/C catalyst with catalysts including more abundant material may lead to lower materials cost associated with the device, and mitigate the CO-poisoning of Pt/C catalyst.
Candidate material include transition metal single-atom catalyst (TM-SAC) which includes Fe or Co single atomic sites coordinated in N-doped carbon.
Carbon capture evaluation was conducted using a cobalt single atom catalyst (Co-SAC). Co-SAC was synthesized based on a hard template method as described in Wu, Z.-Y. et al. Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst. Nat. Commun. 12, 2870, doi:10.1038/s41467-021-23115-x (2021), which allows a production of CO-SAC having high porosity and uniform distribution of metal single atomic sites on the carbon matrix.
Specifically, 1.0 g of o-phenylenediamine (oPD), 0.44 g of COCl2, and 2.0 g of SiO2 nanoparticles (10-20 nm, Aldrich) templates were mixed together by using 20 mL 1.0 M HCl solution. Then the mixed solution was sonicated for 0.5 h and stirred for another 0.5 h. Subsequently, 12 mL of 1.0 M HCl solution, which contains 3.0 g of ammonium peroxydisulphate, i.e. (NH4)2S2O8, was added dropwise into the above mixed solution with vigorous stirring. After polymerization in an ice bath for about one day, the mixture was dried using a rotary evaporator. Then, the dried powder was annealed under Ar atmosphere at 800° C. for 2 h. The product finally was treated by alkaline (2.0 M NaOH) and acid (2.0 M H2SO4) leaching successively to remove SiO2 nanoparticles templates and unstable Co-based species, respectively, to obtain the Co-SAC.
The Co-SAC was shown to have an interconnected 3D porous structure reversely templating from SiO2 nanoparticle templates.
The difference between the performance of the device with Pt/C and So-SAC may be caused by the difference in the active site distribution between Pt/C and So-SAC. As illustrated in
Additional testing was conducted to investigate the device with Co-SAC for its DAC performance. An input gas having 400 ppm CO2 concentration was used to simulate DAC conditions.
Furthermore, the resistance of the device with Co-SAC against CO, SO2 and NO poisoning was evaluated by injecting CO while the device was in operation.
A long-term stability test of the device with Co-SAC was conducted to investigate the suitability of Co-SAC in practical carbon capture applications.
In order to evaluate the carbon capture performance of Co-SAC with an input gas having a low CO2 concentration under a more practical condition, a simulated flue gas (13.9% CO2, 7.8% O2, 76.3% N2, and 2.0% H2O) was prepared to be used as the input gas to the solid electrolyte device. For this evaluation, a tandem reactor system with two identical solid electrolyte device was designed, which is shown in
The carbon capture evaluation of the device under a low O2 concentration, with generated O2 recirculation, is shown in
Factors which may affect the performance of the carbon capture method and the device were investigated.
The effect of the solid electrolyte thickness, or the middle compartment thickness, on the device performance was investigated.
A study was also conducted to evaluate the effect of porous solid electrolyte (PSE) on the device performance.
A study was conducted to evaluate Ni-SAC as the cathode catalyst. Ni-SAC was prepared using the same method as Co-SAC preparation method as previously described, except that 0.405 g of NiCl2·6H2O, and 1.0 g SiO2 were used to synthesize Ni-SAC.
The carbon crossover may occur through the formation of carbonate ions, which requires two-electron transfer per captured CO2 molecule (0.5 CO2/e−). The electron efficiency may be improved by establishing the CO2—H2O2 equilibrium. CO2 may readily react with the HO2− anion from H2O2 to form percarbonate (HCO4−). Therefore, it may be possible to obtain a maximum of 50% increase in electron efficiencies by replacing the 4e−-ORR catalyst with a 2e−-ORR catalyst. Under such reaction scheme, for every two-electron transfer, one OH− and one HO2 may be formed, which can transport 1.5 CO2 gas molecules across the AEM (0.75 CO2/e−), as shown in
Analysis of the obtained CO2 gas was conducted to affirm the carbon capture rate increase demonstrated by the use of Ni-SAC.
Results shown in
Evaluation of ORR catalyst having different H2O2 activities was also conducted using iron single atom catalyst (Fe-SAC) and oxidized carbon black (OCB).
Fe-SAC was prepared as described in Wu, Z.-Y. et al. Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst. Nat. Commun. 12, 2870, doi:10.1038/s41467-021-23115-x (2021).
OCB catalyst was synthesized by adding 2 g of commercially available XC-72 carbon (Vulcan XC-72, Fuel Cell Store) into a three-neck flask with 460 mL 70% HNO3 solution and 140 mL DI water. The mixture was well stirred and refluxed at 80° C. for 24 h. The resulting slurry was washed with water and ethanol after natural cooling until the solution pH reached neutral, and the precipitate obtained was dried overnight under 80° C. in oven. Cathode electrode containing OCB and Fe-SAC were prepared as previously described. The CO2 capture performance of the device with OCB and Fe-SAC was evaluated by introducing an input gas with 13.9% CO2 concentration.
A summary of carbon capture efficiency with respect to H2O2 Faradaic efficiency obtained from various ORR catalysts is shown in
A standard anion MEA cell with a commercial silver nanowire (NW) was used for 2e− CO2 reduction to CO in order to study and systematically quantify the CO2 crossover of a conventional CO2RR electrolyzer. Since the CO2 crossover rates are typically at the same orders of magnitude with CO2 reduction rates, which are usually significantly lower (under small currents) than the CO2 stream flowrates used in CO2RR experiments, this CO2 crossover phenomenon generally do not introduce much error during the quantification of gas product FE. The electrode geometric surface area was 2.5 cm2 and the CO2 upstream (input) flowrate was set at 20 sccm by a mass flow controller (MFC) unless specifically noted.
A schematic of the MEA cell is shown in
The MEA cell included Ag NW catalyst loaded on 2.5 cm2 GDL as a cathode. A PTFE gasket with a 2.5 cm2 window and the AEM membrane (Dioxide Material, X-35) separates the cathode from the Ni foam anode. In the MEA test, the cathode was supplied with 20 sccm of humified CO2 gas using a 50-sccm-range MFC (Alicat Scientific mass flow controller) and the anode was supplied with recycled 0.5M KHCO3 solution via hydraulic pump. The recycled anolyte was constantly bubbled with 50 sccm Ar carrier gas flow and the headspace gas was vented into the GC (SRI 8010C) for anode side gas analysis.
The cathode-side downstream CO2 gas was measured by a gas chromatography (GC) using Ar carrier gas and C2H4 internal integration standard gas. On the anode-side, the downstream oxygen and CO2 were also measured by the GC with 50 sccm of Ar as a carrier gas. The output gas and liquid analyses were conducted as described previously.
The CO2 flow analysis of the cathode side, and anode side are shown in
The natural diffusion of CO2 gas across the AEM can be ruled out because CO2 gas flow on the anode side was not observed when no currents were applied on the cell.
The above result agrees with thermodynamic equilibriums in which hydroxide and CO2 interaction at the catalyst/AEM interface mostly results in carbonate instead of bicarbonate ions due to strong alkaline local pH during CO2RR electrolysis at the cathode. Table 1 shows CO2 reduction half-cell reactions and their corresponding CO2 maximal utilization efficiency assuming 100% product selectivity. Based on this principle, the CO2 crossover problem is exacerbated when higher-value products are targeted (See
COMPARATIVE EXAMPLE 1 demonstrates the challenges of electrolyzers, such as MEA cell, with crossover CO2, in which there have been no effective strategies developed yet to mitigate this carbon loss.
The device and system of the present disclosure were used to successfully recover the “lost” CO2 gas during CO2RR and ORR electrolysis while maintaining outstanding catalytic performances. The above results show that electrolyzers, such as MEA, have poor CO2 utilization efficiency and thus, would be unfeasible in practical applications. Efficient recovery of the carbon was demonstrated with the addition of a porous and ion-conducting solid electrolyte buffer layer to ensure high CO2 utilization efficiencies. This strategy avoids using extra gas separation equipment or energy that can be required to separate crossover CO2 from impurities, especially oxygen. Additional optimization may be conducted by adjusting various factors including the thickness of solid electrolyte layer, and designing different solid ion conductors to improve ion conductions between cathode and anode.
The improvements in cell voltages and electron efficiencies disclosed above may contribute to carbon capture cost reduction. A techno-economic analysis based on reported models and the performance of the device suggests a base cost of $83/ton of captured CO2 before any improvements were implemented. Optimization of the device which includes the use of a thinner PSE layer and higher electron efficiencies, the estimated cost may be reduced to about $58/ton. The economics may be more attractive if the value of generated H2O2 is considered.
The above analysis indicates that the solid electrolyte carbon capture reactor of the present disclosure represents a competitive, promising and sustainable strategy for carbon management. For example, ORR/OER redox couple presents about at least 500 mV onset voltage as well as sluggish Tafel slopes, which can be avoided when switching to other facile redox couples in different application scenarios, such as hydrogen evolution/hydrogen oxidation reaction (HER/HOR), organic and inorganic molecule redox couples, etc. In addition, a thinner solid electrolyte layer can be fabricated using more advanced machining tools or 3D printer, which can further improve the ohmic drop of our device for better cell voltages. Other operation parameters such as temperature for better reaction kinetics and pressure for better mass transports could also be implemented for different application scenarios. Taking the above factors into consideration, the carbon capture cost may be brought down to approximately $33/ton.
Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112(f) for any limitations of any of the claims, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.
This invention was made with government support under Grant No. 2029442 awarded by the National Science Foundation. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2023/018650 | 4/14/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63363026 | Apr 2022 | US |
