Single atom catalysts anchored on three-dimensional mesoporous supports for carbonate electrolysis to syngas

Abstract

Provided are electrolyzers for reducing CO2 to syngas, methods of reducing CO2 to syngas, and catalysts which may be used in the electrolyzers and methods. In embodiments, an electrolyzer comprises a cathode comprising a single metal atom catalyst comprising single metal atoms distributed within a mesoporous carbon matrix, the cathode in contact with a catholyte comprising carbonate ions; an anode in electrical communication with the cathode; and a bipolar membrane configured to generate in situ CO2 in the catholyte. Doped single metal atom catalysts are also provided.

Description

Atty. Dkt. No.00100-0360-PCT SINGLE ATOM CATALYSTS ANCHORED ON THREE-DIMENSIONAL MESOPOROUS SUPPORTS FOR CARBONATE ELECTROLYSIS TO SYNGAS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 612,029 that was filed December 19, 2023, the entire contents of which are incorporated herein by reference. REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under N00014-22-1-2690 awarded by the DOD. The government has certain rights in the invention. BACKGROUND

[0003] Direct air capture (DAC) systems typically use alkali hydroxide to capture CO2 as carbonate, followed by pure CO2generation through a high-temperature calcium caustic process to release CO2 and regenerate base. Carbonate electrolyzers convert aqueous carbon capture solutions into valuable products (e.g., syngas) that can bypass energy-intensive CO2regeneration, circulation, and purification steps. However, such systems are limited by the low CO faradaic efficiency (FE) of <25% on Ag catalysts in bipolar membrane-based electrolyzers. SUMMARY

[0004] Provided are electrolyzers for reducing CO2to syngas, methods of reducing CO2to syngas, and catalysts which may be used in the electrolyzers and methods.

[0005] The present disclosure includes an Example 1 below demonstrating that single atom catalysts anchored on three-dimensional (3D), mesoporous supports can modify local CO2 transport and enhance its utilization for the electrochemical conversion of CO2, captured in the form of carbonate, into syngas. Through a coordination-condensation strategy, single- atom Ni anchored onto nitrogen-assembled carbon (Ni-NAC) in a three-dimensional cylindrical configuration was obtained, which featured a channel length of 5-10 μm and pore diameters of 3-5 nm. The ordered mesoporous structure facilitates effective exposure of metal sites and enhances mass transfer properties. Over 42% FE for CO with >99% carbonAtty. Dkt. No.00100-0360-PCT utilization at 100–300 mA cm−2and an energy efficiency (EE) of 54% at 100 mA cm−2were obtained. Such a system obviates the energy requirement to regenerate / separate lost / unreacted reactants.

[0006] As noted in Example 1, other catalysts, including doped single metal atom catalysts (e.g., phosphorous (P) doped Ni-NAC catalysts) and single metal atom catalysts fabricated using a variety of 3D mesoporous supports (e.g., KIT-6 and MCM-48) were synthesized, characterized, and used to electrochemically reduce CO2. These catalysts are also encompassed by the present disclosure, including for use with the disclosed electrolyzers and syngas production methods.

[0007] Example 2 is an additional demonstration of the present disclosure and includes results involving single metal atom catalysts fabricated using additional 3D mesoporous supports, including MCM-48.

[0008] In embodiments, an electrolyzer comprises a cathode comprising a single metal atom catalyst comprising single metal atoms distributed within a mesoporous carbon matrix, the cathode in contact with a catholyte comprising carbonate ions; an anode in electrical communication with the cathode; and a bipolar membrane configured to generate in situ CO2 in the catholyte.

[0009] In embodiments, an electrolyzer comprises a cathode comprising a Ni-NAC catalyst comprising single nickel atoms distributed within a mesoporous carbon nitride matrix, the cathode in contact with an aqueous alkali metal carbonate catholyte comprising carbonate ions; an anode in electrical communication with the cathode; a bipolar membrane configured to generate in situ CO2in the catholyte; and an interposer layer between the cathode and the bipolar membrane.

[0010] In embodiments, a doped single metal atom catalyst comprises a dopant element and single metal atoms distributed within a mesoporous carbon matrix.

[0011] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.Atty. Dkt. No.00100-0360-PCT BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Illustrative embodiments of the disclosure will hereafter be described with reference to the accompanying drawings.

[0013] FIG.1 shows a schematic of syngas production through electrochemical conversion of CO2 captured from air. In an integrated process (bottom), carbonate is directly fed into a bipolar membrane (BPM)-based electrolyzer for syngas generation, bypassing the CO2 regeneration and circulation processes that is used in a sequential process (top).

[0014] FIGS.2A-2D show the synthesis and characterization of Ni-NAC catalysts. (FIG. 2A) Schematic illustration of the two-step synthesis of Ni-NAC catalysts. (FIG.2B) HRTEM images. (FIGS.2C-2D) HAADF images. Other EDS mapping images of the Ni-NAC catalyst were obtained (data not shown).

[0015] FIGS.3A-3D show results for the electrochemical generation of syngas from the carbonate liquid on Ni-NAC catalysts. (FIG.3A) Schematic illustration of the carbonate electrolyzer. The electrolyzer was fed with 1.5 M K2CO3as the catholyte. Carbonate passes through the hydrophilic carbon paper, the catalyst layer, and the hydrophilic interposer layer (thickness: 135 μm) through convection and diffusion. CO32–directly reacted with H+from the BPM to generate i-CO2. The i-CO2 passes back from the membrane interface and goes through the interposer layer to the catalyst layer for CO generation. (FIG.3B-3C) Faradaic efficiency of CO on Ni-NAC catalysts made using various carbonization temperatures and precursor amounts. (FIG.3D) Comparison of Faradaic efficiency of CO on Ni-NAC catalyst with Ag catalysts.

[0016] FIG.4 shows structures of dopant precursors.

[0017] FIG.5 shows a carbonate reduction to syngas system.

[0018] FIG.6A illustrates the local environment of a carbonate electrolyzer, including the BPM with cation exchange layer (CEL) and anion exchange layer (AEL), local transport of CO2and CO32-, Ni-PPTL, and various reactions for CO2loss and conversion. FIGS.6B and 6C show a comparison between the CO2 transport and conversion in two systems: a carbonate electrolyzer operating in a hydrophilic environment (FIG.6B) and a conventional CO2 electrolyzer operating in a hydrophobic environment (FIG.6C). The conventional CO2electrolyzer includes a hydrophobic porous transport layer (PTL) consisting of a hydrophobic gas diffusion layer (GDL) and a microporous layer (MPL) for CO2transport, along with aAtty. Dkt. No.00100-0360-PCT catalyst layer (CL) for CO2 conversion. In reactive capture system in the present disclosure, a Ni-PPTL is used that incorporates Ni single sites and hydrophilic porous carbon-nitrogen matrix as the porous transport layer, enabling efficient i-CO2 transport and conversion within the pores.

[0019] FIGS.7A and 7B show pore size distribution in Ni-PPTL synthesized using different mesoporous templates. FIG.7C shows Faradaic efficiency to CO and percentage of micropore as a function of cathode electrode at 200 and 300 mA / cm2 in carbonate electrolyzer with BPM. FIG.7D shows total capacitance (Cdl) normalized by the BET surface area. FIG.7E shows the calculated τo under diluted K2CO3electrolytes. Relaxation time constant (τo) is calculated from the peak frequency (fo) of imaginary part of complex capacitance (Cim) using the relation τo = (2πfo)–1. FIG.7F shows single pass conversion of CO2 in gas-CO2-fed, MEA-based electrolyzer. AEM was used as the ion exchange membrane.

[0020] FIGS.8A-8D shows optimization of electrochemical performance and energy consumption for carbonate electrolysis on Ni-PPTL. (FIG.8A) Schematic illustration of BPM-based electrolyzer for carbonate electrolysis. (FIG.8B) Faradaic efficiency to CO and H2 on Mul-MMP catalysts at 100-400 mA / cm2. (FIG.8C) Cell voltage (left Y-axis) and energy efficiency (right Y-axis) for carbonate electrolysis (1.5 M K2CO3) at different cell configurations. The energy efficiency is calculated using experimental data for a system that employed the optimized BPM and a NiFeP anode. (FIG.8D) Cell voltage (left Y-axis) and Faradaic efficiency to CO (right Y-axis) for 50 hours of electrolysis at 200 mA / cm2with an optimum cell configuration: Mul-MMP cathode, SnO2-BPM, and NiFeP anode.

[0021] FIGS.9A-9D show scale up of carbonate electrolysis system. (FIG.9A) Schematic illustration of scaled-up flow plates. The channel depth is 1 mm, width is 1 mm, and the rid distance is 3.5 mm. The active area length is 50 mm, resulting in an electrode active area of 25 cm². (FIG.9B) Faradaic efficiency to CO (left Y-axis) and CO concentration (right Y-axis) at the cathode outlet. (FIG.9C) CO production rate (mmol h⁻¹) and (FIG.9D) CO2 removal amount during 5 hours of electrolysis at currents of 1.25 A and 2.5 A.Atty. Dkt. No.00100-0360-PCT DETAILED DESCRIPTION

[0022] Electrolyzers are provided which are configured to generate CO2 in situ (i.e., i- CO2) in an electrolyte contained within the electrolyzer and to reduce the i-CO2to products (e.g., syngas) using a single metal atom catalyst. (See FIG.3A.) Such an electrolyzer comprises a cathode comprising the single metal atom catalyst, an anode, and a bipolar membrane between the cathode and the anode.

[0023] The single metal atom catalyst comprises single metal atoms (as distinguished from a collection of multiple metal atoms, e.g., metal nanoparticles) which are distributed within a mesoporous carbon matrix. The mesoporous carbon matrix may comprise a non- carbon element, e.g., nitrogen, in addition to carbon. Methods for synthesizing the single metal atom catalysts are described in the Examples, below. (See FIG.2A.) Briefly, the methods involve a templated metal coordination-condensation scheme comprising combining a metal precursor (e.g., a metal acetylacetonate), a chelating precursor (e.g., a diamine such as ethylene diamine), a carbon precursor (e.g., CCl4), and a mesoporous template (e.g., a mesoporous silica template such as SBA-15) under conditions to induce condensation reactions on and within the mesoporous template to produce a condensation product; carbonizing the condensation product; and removing the mesoporous template to provide the single metal atom catalyst. Conditions being used during the synthesis method, including relative amounts of reactants (e.g., metal precursor), temperatures (e.g., carbonization temperature), etc., may be adjusted to tune properties of the single metal atom catalyst, including to achieve a desired faradic efficiency for CO and / or energy efficiency.

[0024] Characteristics of the single metal atom catalysts include high porosity and surface area; uniform, ordered morphology (generally corresponding to the mesoporous template being used) (see FIGS.2B, 2C); and individual metal atoms bound within a graphitic carbon network (which may be a graphitic carbon nitride network) (see FIG.2D). These and other characteristics are further described in the Examples, below. Regarding porosity and morphology, Brunauer–Emmett–Teller (BET) surface area and average pore size analysis as described in the Examples below may be used. In embodiments, the pores of the single metal atom catalysts may comprise or consist of mesopores (pores having an average diameter in a range of from 2 to 50 nm). In embodiments, the pores of the single metal atom catalysts may comprise or consist of both mesopores and micropores (pores having an average diameter of less than 2 nm). In embodiments, a majority of the pores of theAtty. Dkt. No.00100-0360-PCT single metal atom catalysts are micropores. In embodiments, the pores of the single metal atom catalysts comprise or consist of micropores.

[0025] Single metal atom catalysts formed using nitrogen-containing chelating precursors may be referred to herein as M-NAC catalysts, wherein “M” refers to the single metal atoms and “NAC” refers to the nitrogen assembly carbon network. A variety of metals may be used, including transition metals such as Ni, Fe, Co, Cu, Pt, Pd, Ru, Rh, and combinations thereof.

[0026] An illustrative synthesis method and the resulting Ni-NAC catalyst is shown in FIG.2A along with images of the Ni-NAC catalyst shown in FIGS.2B-2D.

[0027] The synthesis methods may be used with a variety of mesoporous templates to provide single metal atom catalysts having different morphologies, including different channel lengths and pore diameters. Besides SBA-15 which was employed in Example 1, below, other mesoporous templates which may be used include derivatives of SBA-15, KIT- 6, and MCM-48, as shown in Example 2.

[0028] The synthesis methods may be used to provide doped single metal atom catalysts. This may be accomplished by including a dopant precursor to the reactants. The dopant precursor comprises a dopant element. The dopant element is a non-carbon element as distinguished from another non-carbon element that may be present in the mesoporous carbon matrix. The dopant element may be a non-metal element. Illustrative dopant elements include, e.g., sulfur, boron, phosphorous, and combinations thereof. The dopant precursor may also include both the dopant element and the desired metal. For example, dichloro[1,2- bis(diphenylphosphino)ethane]nickel is an illustrative phosphorous nickel precursor which may be used alone or with another metal precursor, e.g., nickel acetylacetonate. Illustrative dopant precursors are shown in FIG.4. The type and relative amount of dopant precursor may be adjusted to tune properties of the doped single metal atom catalyst.

[0029] The single metal atom catalyst may be provided as an ink (which may further comprise other components, e.g., an ionomer) and coated onto a substrate to form the cathode of the electrolyzer. The cathode is in contact with a catholyte comprising carbonate ions (CO32-). The catholyte may be an aqueous alkali metal carbonate (e.g., K2CO3) solution.

[0030] The bipolar membrane between the anode and the cathode of the electrolyzer is configured to generate the i-CO2 in the catholyte. This may be accomplished by the bipolar membrane being configured to dissociate water contained within the electrolyzer, thereby generating protons and hydroxide ions. On the cathode side of the electrolyzer, the protonsAtty. Dkt. No.00100-0360-PCT may combine with the carbonate ions to generate the i-CO2 in the catholyte. As described in the Examples, below, the bipolar membrane may comprise a catalyst to facilitate water dissociation.

[0031] The anode of the electrolyzer may be configured to facilitate the formation of oxygen (O2) from hydroxide ions present in the anolyte (including those generated by the bipolar membrane). Various anodes (e.g., NiFeOx) and anolytes (e.g., alkali metal hydroxides such as KOH) capable of achieving oxygen evolution may be used.

[0032] The electrolyzer may further comprise an interposer layer between the cathode and the bipolar membrane, the interposer layer configured to suppress hydrogen evolution and to facilitate transport of the i-CO2to the cathode.

[0033] An illustrative electrolyzer is shown in FIG.3A. The electrolyzer comprises a cathode comprising a Ni-NAC catalyst, the cathode in contact with an aqueous K2CO3solution as the catholyte (fed to the electrolyzer via a feed inlet). The carbonate ions in the catholyte react with the protons from the bipolar membrane to generate i-CO2 in the catholyte. Generating a potential difference between the anode and the cathode induces reduction of the i-CO2 at the cathode as catalyzed by the Ni-NAC catalyst thereon. The products of the CO2reduction reaction (e.g., syngas) may be collected via an outlet.

[0034] The disclosed electrolyzers may be a component of a larger system as illustrated in FIG.5.

[0035] Methods for producing syngas from i-CO2are also provided. Such a method comprises generating i-CO2 in a catholyte comprising carbonate ions, the catholyte in contact with a cathode of an electrolyzer, the cathode comprising a single metal atom catalyst; and reducing the i-CO2 to syngas at the cathode. The methods may be carried out using any of the disclosed electrolyzers and any of the disclosed single metal atom catalysts.

[0036] Also encompassed are the single metal atom catalysts themselves, including the doped single metal atom catalysts, e.g., P-doped Ni-NAC. Use of such single metal atom catalysts, e.g., P-doped Ni-NAC, is not limited to the disclosed electrolyzers and the disclosed syngas production methods. EXAMPLES

[0037] Example 1Atty. Dkt. No.00100-0360-PCT

[0038] Introduction

[0039] Direct air capture (DAC) provides the advantage of a distributed, dependable, and environmentally friendly supply of CO2. Typically, an alkaline solution flows downward to capture CO2 in the form of K2CO3. Subsequently, the pure gas-phase CO2 is liberated by additional energetic steps of dry and calcination of the carbonate salt. The valorization of gaseous CO2 into valuable products (e.g., electrochemical CO2 reduction) introduces further energy losses and system complexity, largely due to the low utilization of CO2. By contrast, a reactive capture system integrates CO2 capture with CO2 upgrade in a single device, bypassing the energy-intensive CO2regeneration and purification steps (FIG.1). Electrochemically generating syngas (CO + H2) from captured CO2 is regarded as the most promising product. Syngas serves as a common raw material employed in generating hydrocarbons and oxygenates through processes such as methanol pathways and Fischer- Tropsch synthesis.

[0040] The integrated capture-and-electrolysis system relies on H+at the membrane interface to react with CO32–to form in situ CO2 (i-CO2), which serves as the reactive species for subsequent conversion into carbon products. The carbonate stream is fed into an electrochemical cell that is equipped with a bipolar membrane (BPM), where H+is generated from the water dissociation in the middle compartment. However, syngas production in the BPM-based electrolyzer is limited by the low CO Faradaic efficiency (FE) below 25% on Ag catalysts.

[0041] In this Example, a Ni single-atom catalyst anchored on nitrogen assembly carbons (Ni-NACs) was implemented for carbonate reduction to syngas. Taking advantage of the ordered mesoporous architecture with a three-dimensional architecture and abundant single- atom Ni sites, the mass transport of CO2 was largely enhanced. Such a catalyst design principle substantially facilitates CO2-to-CO conversion and achieves >42% CO FE across a current density range of 100-300 mA cm–2. Ni-NAC also exhibited ~200 mV cell voltage decreases as compared to Ag catalysts. Furthermore, by employing system engineering optimization, an energy efficiency (EE) of 54% at 100 mA cm–2was achieved for the conversion of air to syngas (with an H2:CO = 2:1). Such a catalyst and system enabled the carbonate electrolyzer as an energy-efficient and cost-effective route for carbon capture and utilization.Atty. Dkt. No.00100-0360-PCT

[0042] The catalyst design principle may be applied to other metal centers (e.g., Fe, Co, Cu, Pt, Pd, Ru, and Rh) and support templates (e.g., KIT-6, MCM-48 and SBA-15) for various applications.

[0043] Also encompassed are the catalysts support themselves, including the pore size, channel length and pore structure manipulated via changing templates (e.g., KIT-6, MCM-48 and SBA-15). Use of such pore-size-, channel-length- and pore-structure-adjustable carbon support, is not limited to the disclosed electrolyzers and the disclosed syngas production methods.

[0044] Experimental Methods

[0045] Chemicals and Materials

[0046] Nickel(II) acetylacetonate (95%), carbon tetrachloride (99.9%), ethylenediamine (99.5%), tetraethyl orthosilicate (TEOS, 98%), oleylamine (OAm,>70%), potassium carbonate (ACS reagent, ≥99.0%), and potassium hydroxide (ACS reagent, ≥85) were purchased from Sigma-Aldrich. Hydrofluoric acid (trace metal grade), hydrochloric acid (trace metal grade), and nitric acid (trace metal grade) were purchased from Fisher Scientific. The bipolar membrane (Fumatech FBM) and hydrophilic carbon paper (Freudenberg H23) were purchased from Fuel Cell Store. Ni foam (1.6 mm thick) was purchased from MTI Corporation. All chemicals were used as received without further purification. Deionized (DI) water (18.2 MΩ cm, Barnstead™ E-Pure™) was used for all experiments in this Example.

[0047] Catalyst Synthesis

[0048] The Ni-NAC was synthesized as follows. Typically, in the synthesis of SBA-15: Six grams of Pluronic P123 was first dissolved in 180 mL 2 M hydrochloric solution at 40 °C with the help of stirring (500 rpm) in a 500 mL sealed polypropylene bottle. After being dissolved, 12.75 g of TEOS was added, and the solution was kept stirring for 20 h. The composition of the achieved gel was: 1 TEOS: 0.017 P123: 5.68 HCl: 197 H2O. After 20 h, the whole bottle was kept in an oven at 100 °C for 24 h under static condition. The white precipitant was recovered by Büchner filtration, washed with ultrahigh purity water / methanol 1:1 volume ratio twice, and was kept in the hood for 3 days for air dry. The obtained solid was heated to 550 °C under air at a heating rate of 2 °C min–1and calcined for 5 h.Atty. Dkt. No.00100-0360-PCT

[0049] For synthesis of Ni-NAC catalysts: The Ni (II) acetylacetonate was first added into the solution of ethylenediamine (1.80 g) and carbon tetrachloride (4.00 g), followed by adding SBA-15 (0.80 g). “Caution: the metal precursor may catalyze the condensation of carbon tetrachloride and ethylenediamine with the release of HCl gas.” The mixture was then heated in an oil bath at 90 °C for 16 h for condensation before the oil bath temperature increased to 120 °C for 4 h to remove the uncondensed ethylenediamine and carbon tetrachloride. The obtained powders were calcined under Ar flow, with temperature raised at a ramping rate of 3 °C min–1and further maintained at 800 °C for 2 h. The achieved black powder was then etched with 5 wt.% HF solution to remove the SBA-15. The catalyst was recovered via centrifuge, washed with deionized water until the pH reached 7. The catalyst was further dried at 100 °C for future usage.

[0050] Electrochemical measurements

[0051] The flow electrolyzer contains two stainless steel flow-field plates with serpentine channels, PTFE and silicone gaskets, and the membrane electrode assembly (MEA), which contains two electrodes and a membrane, and was formed after assembling the cell hardware. The catholyte and anolyte were circulated by peristaltic pumps (INTLLAB) at 15 ml min–1. The applied current was controlled by an Autolab potentiostat / galvanostat. The membrane used to separate catholyte and anolyte was a commercial (Fuel Cell Store) or custom- designed bipolar membrane (BPM). The as-prepared Ni-NAC catalyst was used as the cathode. A piece of filter membrane was inserted as the interposer layer between the cathode and cation exchange layer (CEL). The thickness of the interposer layer was controlled at 135 μm and the pore size was 8 μm (mixed cellulose esters (MCE) membrane, hydrophilic). The catholyte was 1.5 M K2CO3, and the anolyte was 1 M KOH. All experiments were performed at room temperature.

[0052] The catalyst ink was prepared by dispersing Ni-NAC in the 2-propanol solvent with added Nafion ionomer by ultrasonication. The ink needs to be well-sonicated for a good dispersion of the catalyst. The mass ratio of the Ni-NAC and ionomer was 9:1. The ink was then airbrushed onto the substrate to the final loading of ~1.5 mg cm–2.

[0053] NiFeOx electrode as an anode was prepared as follows. Ni foam was first cleaned with 6 M HCl and DI-water for 15 min under sonication. Then, a 40 mL solution with 4 mmol NH4F, 10 mmol urea, 2 mmol Ni(NO3)2^6H2O, and 2 mmol Fe(NO3)3^9H2O was prepared and transferred to a 50 mL Teflon-lined stainless steel autoclave. The hydrothermalAtty. Dkt. No.00100-0360-PCT growth of the hydroxides on Ni foam was performed at 120 °C for 6 hours with a heating rate of 3 °C min−1, followed by sonication in DI-water and drying in the oven at 80°C.

[0054] Product analysis

[0055] The gas products (H2 and CO) were quantified by GC (Shimadzu 2014, PerkinElmer Clarus 580) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) equipped with a Methanizer. The calibration curve was established by analyzing the standard calibration gases with different concentrations (10–10,000 ppm). Argon (100 mL min-1) was purged as the carrier gas to carry the gas products out of the system for quantification.

[0056] The rate of H2 / CO generation (r, mol s−1) for each cycle was calculated by the following equation: ൌ^^ ൈ 10 ൈ ^^^^^ ൈ10ି^^^ ି^^ ^^^^ where c is the H2 / CO concentration (ppm); v is the volumetric flow rate of the inlet gas (100 ml min−1); p is the ambient pressure (p = 1.013 × 105Pa); R is the gas constant (R = 8.314 J mol−1K−1); T is room temperature (293.15 K). The total amount of gas (mol) was calculated by integrating the plot of H2 / CO production rate (mol s−1) vs. reaction time (s).

[0057] The Faradaic efficiency (FEi) can be calculated by the following equation: ^^ ^^ ^^ ^^^^ ൌ^ ^^^^ൈ 100%here n0is initial moles of reactant; n is the moles of reactant after electrolysis; niis the moles of product i; zi is the number of electrons transferred for one product molecule; F is the Faraday constant (96,485 C mol−1); Q is the total charge passed through the electrolytic cell; t is the electrolysis time (s).

[0058] Materials characterization

[0059] Aberration corrected HAADF STEM imaging was performed using a probe corrected Thermo Fisher (FEI) Titan Themis. Powder X-ray diffraction (XRD) was carried out on a Bruker D8A25 diffractometer with Cu Kα radiation (λ = 1.54184 Å). N2 physisorption was performed using an auto-adsorption analyzer (Micromeritics, 3Flex) at - 196 °C. Elementary analysis was conducted using a Perkin Elmer 2100 Series II CHN / S Analyzer. Inductively coupled plasma mass spectroscopy (ICP-MS) for metal loadings wasAtty. Dkt. No.00100-0360-PCT performed using a Thermo Fisher Scientific X Series 2 spectrometer. X-ray photoelectron spectrometry (XPS) was recorded on a PerkinElmer PHI ESCA system with Physical Electronics (PHI).

[0060] Results and Discussion

[0061] Catalyst preparation and characterization

[0062] The Ni-NAC catalyst was prepared by using a coordination-condensation method, followed by carbonization (FIG.2A). Ni precursor (Nickel(II) acetylacetonate) was first dispersed in the ethylenediamine solution, which served as an ideal N source for the carbon materials and ensured a high N / C ratio in the final catalyst, followed by the slow addition of carbon tetrachloride. This allowed in situ growth of a templated catalyst polymer net where Ni is coordinated with multiple N, creating single metal sites that favor binding and activation of CO2. During the following condensation process, the silica template of SBA-15 ensured the mesopores morphology of the catalyst with the cylindrical pore channels. The integration of isolated Ni sites into NAC support was then obtained by a carbonization process.

[0063] Scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) images (FIG.2C) showed that the single-atom Ni-NAC possesses an ordered rod- shaped pore morphology with the channel length of 5-10 μm and pore diameters of 3.4–5.5 nm. The formation of single atom Ni is also visualized by the STEM-HAADF image (FIG. 2D, white circles). STEM-energy-dispersive X-ray spectroscopy (STEM-EDS) mapping (data not shown) suggested a uniform anchor of Ni on the NAC support. ICP and XRD further indicated the Ni loading of 1.7%, without any Ni nanoparticle formation. Brunauer–Emmett– Teller (BET) analysis showed its specific surface area of 720 m2 / g.

[0064] Electrochemical performance on Ni-NAC catalysts.

[0065] The Ni-NAC catalysts with mesoporous architecture for CO2binding are key factors in enhancing local i-CO2 transport and its subsequent electrochemical conversion in a BPM-based electrolyzer (FIG.3A). An interposer layer was used between the cathode and membrane to create an appropriate local environment for suppressing hydrogen evolution reaction (HER) and favorable i-CO2transport.

[0066] It was observed that the carbonization temperature and Ni precursor amount during the synthesis of Ni-NAC are crucial in affecting the CO FE (FIGs.3B-3C). ThroughAtty. Dkt. No.00100-0360-PCT systematic testing, a maximum CO FE of 44% at 100 mA cm−2was obtained with a carbonation temperature of 800 °C and Ni precursor amount of 50 mg. The high performance of CO FE can be maintained at >42% in the current density range of 100-300 mA cm−2, which is attributed to the highly mesoporous architecture of the catalysts. The elevated carbonization temperature (> 800 °C) led to a reduction in the N content within the Ni-NAC catalysts, thereby contributing to an enhancement in electronic conductivity. A lower carbonization temperature and higher precursor amount leads to formation of Ni nanoparticles, which mainly favors the hydrogen evolution reaction (HER). A much lower precursor amount decreased the Ni loading, resulting in insufficient active sites for i-CO2 utilization. In sum, the mesoporous three-dimensional structure allows single metal sites to be exposed to reactants and remain dispersed during the reaction; thus, improving catalyst reactivity and stability.

[0067] To increase the energy efficiency of the system, the cell voltage of the electrolyzer was reduced. In the carbonate electrolyzer used here, the major potential drop originates from the BPM, because of the sluggish kinetics for water dissociation (WD). By implementing a P25-TiO2 WD catalyst, a significant reduction in membrane voltage loss was achieved, and therefore, a sudden decrease in cell voltage. In the cell configuration using P25 TiO2-BPM, Ni-NAC cathode, and NiFeOx anode, a cell voltage of 2.55 V at 100 mA cm-2was obtained. This enabled a syngas energy efficiency of 53%.

[0068] The excellent CO FE (>42%) on Ni-NAC outperformed that using Ag catalysts (CO FE<20%) in BPM-based electrolyzer (FIG.3D). The impressive performance of Ni- NAC is attributed to its mesoporous carbon structure, which facilitates the transport and retention of i-CO2 within the gas channel pores. Additionally, Ni single sites are effectively and uniformly distributed within the pores, enhancing the accessibility and utilization of localized CO2.

[0069] Additional experiments were conducted as described above to provide other catalysts, including doped single metal atom catalysts (e.g., phosphorous (P) doped Ni-NAC catalysts) and single metal atom catalysts fabricated using a variety of 3D mesoporous supports (e.g., KIT-6 and MCM-48). These catalysts were synthesized, characterized, and used as described above to electrochemically reduce CO2.Atty. Dkt. No.00100-0360-PCT

[0070] Additional information, including experimental data, may be found in U.S. provisional patent application number 63 / 612,029 that was filed December 19, 2023, the entire contents of which are incorporated herein by reference.

[0071] Example 2

[0072] Experimental Methods

[0073] Chemicals and Materials

[0074] Nickel(II) acetylacetonate (95%), carbon tetrachloride (99.9%), ethylenediamine (99.5%), tetraethyl orthosilicate (TEOS, 98%), oleylamine (OAm,>70%), potassium carbonate (ACS reagent, ≥99.0%), and potassium hydroxide (ACS reagent, ≥85) were purchased from Sigma-Aldrich. Hydrofluoric acid (trace metal grade), hydrochloric acid (trace metal grade), and nitric acid (trace metal grade) were purchased from Fisher Scientific. The bipolar membrane (Fumatech FBM) and hydrophilic carbon paper (Freudenberg H23) were purchased from Fuel Cell Store. Ni foam (1.6 mm thick) was purchased from MTI Corporation. All chemicals were used as received without further purification. Deionized (DI) water (18.2 MΩ cm, Barnstead™ E-Pure™) was used for all experiments in this work.

[0075] Synthesis of different hard templates

[0076] Synthesis of SBA-15. For the SBA-15 template synthesis, Pluronic P123 (4.0 g) was dissolved in 120 mL of 2.0 M HCl solution at 37oC in a 500 mL polypropylene bottle. Once fully dissolved, TEOS (8.5 g) was added, and the solution was stirred for 20 hours at 37oC. The mixture was then transferred to an oven and kept at 100 °C for 24 hours under static conditions. The resulting white precipitate was recovered by filtration, washed twice with DI water / methanol (1:1, v / v), and air-dried in a hood for three days. The dried solid was calcined at 550oC for 5 hours in air with a ramping rate of 2oC / min. For the synthesis short channel SBA-15, in a 500 mL polypropylene bottle, Pluronic P123 (2.4 g), NH₄F (6.9 mg), and 1.94 M HCl (80 mL) were added. Once fully dissolved, a pre-mixed solution of TEOS (5.5 mL) and heptane (0.663 g) was added to the bottle, and the mixture was stirred for 4 minutes before being left under static conditions for 20 hours. The mixture was then transferred to an oven and kept at 100 °C for 24 hours under static conditions. The resulting white precipitate was recovered by filtration, washed twice with a 1:1 (v / v) mixture of DI water and methanol, and air-dried in a hood for 3 days. The dried solid was calcined at 550oC for 5 hours in air with a ramping rate of 2oC / min.Atty. Dkt. No.00100-0360-PCT

[0077] Synthesis of MCM-48. For the MCM-48 template synthesis, in a 500 mL round- bottom flask, OTAB (0.678 g) and F-127 (2.538 g) were dissolved in a mixture of ethanol (43.0 g), water (129.8 g), and ammonia solution (12.24 g, 30–33% NH₃ in H₂O). The mixture was stirred vigorously at 850 rpm until fully dissolved. Subsequently, TEOS (1.8 g) was quickly added to the flask. Upon addition, the mixture was stirred continuously for approximately 1.5 minutes, during which it changed from colorless to light blue. The reaction mixture was then left undisturbed at room temperature for 24 hours. The resulting solid was collected by centrifugation at 8000 rpm for 10 minutes, washed three times with ethanol (35 mL each), and dried under vacuum at room temperature overnight. The obtained powder was then calcinated at 550oC for 6 hours under air at a ramping rate of 5oC / min.

[0078] Synthesis of KIT-6. For the KIT-6 template synthesis, Pluronic P123 (4.0 g) was dissolved in a mixture solution of 144 mL of DI water and 7.9 mL of 35 wt. % hydrochloric acid. After which, 4.0 mL of n-butanol was added. This mixture was then stirred vigorously for 1 hour at 35 ºC. Following this, 8.6 mL of TEOS was added, and the solution was stirred continuously for another 24 hours. The mixture was then subjected to hydrothermal synthesis at 100 ºC for 24 hours. Afterward, the resultant solid was filtered and washed with DI water. The obtained solid was then calcinated at 550 ºC for 6 hours.

[0079] Synthesis of M-NACs with different hard templates

[0080] To a 50 mL round bottom flask, 1.8 g of ethylenediamine was introduced, followed by the addition of 25 ~ 100 mg of nickel (II) acetylacetonate. The mixture was stirred for 5 minutes before 4.0 g of carbon tetrachloride was added and stirring continued for an additional 5 minutes. Subsequently, 0.8 g of silica template (SBA-15, KIT-6, or MCM-48) was introduced, and the mixture was refluxed at 90°C for 16 hours. After which, the mixture was dried at 130°C to evaporate any remaining carbon tetrachloride and ethylenediamine. The resultant residue was then pyrolyzed under an Ar atmosphere at 800 °C for 2 hours, with a ramping rate of 3 °C / min. The black powder obtained was dispersed in a solution of 5 wt.% HF and 10 wt.% HCl (35 mL) and stirred for 24 hours. The catalyst was collected by centrifugation, which was then washed with deionized water (> 10 cycles) until a neutral pH was achieved. Finally, the black powder was dried at 80°C overnight and stored for future use.

[0081] Electrochemical measurementsAtty. Dkt. No.00100-0360-PCT

[0082] The flow electrolyzer contains two stainless steel flow-field plates with serpentine channels, PTFE and silicone gaskets, and the MEA, which contains two electrodes and a membrane, and was formed after assembling the cell hardware. The catholyte and anolyte were circulated by peristaltic pumps (INTLLAB) at 15 ml min–1. The applied current was controlled by an Autolab potentiostat / galvanostat. The membrane used to separate catholyte and anolyte was a commercial (Fuel Cell Store) or custom-designed BPM. The as-prepared Ni-NAC catalyst was used as the cathode. A piece of filter membrane was inserted as the interposer layer between cathode and cation exchange layer (CEL). The thickness of the interposer layer was controlled at 135 µm and the pore size was 8 µm (mixed cellulose esters (MCE) membrane, hydrophilic). The catholyte was 1.5 M K2CO3, and the anolyte was 1 M KOH. All experiments were performed at room temperature.

[0083] The catalyst ink was prepared by dispersing Ni-NAC in 2-propanol solvent with added Nafion ionomer by ultrasonication. The ink needs to be well-sonicated for a good dispersion of catalyst. The mass ratio of the Ni-NAC and ionomer was 9:1. The ink was then airbrushed onto the substrate to the final loading of ~1.5 mg cm–2.

[0084] NiFeOxelectrode as anode was prepared as follows. Ni foam was first cleaned by 6 M HCl and DI-water for 15 min under sonication. Then, a 40 mL solution with 4 mmol NH4F, 10 mmol urea, 2 mmol Ni(NO3)2^6H2O, and 2 mmol Fe(NO3)3^9H2O was prepared and transferred to a 50 mL Teflon-lined stainless steel autoclave. The hydrothermal growth of the hydroxides on Ni foam was performed at 120 °C for 6 hours with a heating rate of 3 °C min−1, followed by sonication in DI-water and drying in the oven at 80°C.

[0085] Product analysis

[0086] The gas products (H2 and CO) were quantified by GC (Shimadzu2014, PerkinElmer Clarus 580) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) equipped with a Methanizer. The calibration curve was established by analyzing the standard calibration gases with different concentrations (10–10,000 ppm). Argon (100 mL min-1) was purged as the carrier gas to carry the gas products out of the system for quantification.

[0087] The rate of H2 / CO generation (r, mol s−1) for each cycle was calculated by the following equation: r = c ^ 10−6^ [Pv̇ ^ 10−6 / (RT)]Atty. Dkt. No.00100-0360-PCT

[0088] where c is the H2 / CO concentration (ppm); V̇ is the volumetric flow rate of the inlet gas (100 ml min−1); p is the ambient pressure (p = 1.013 × 105Pa); R is the gas constant (R = 8.314 J mol−1K−1); T is the room temperature (293.15 K). The total amount of gas (mol) was calculated by integrating the plot of H2 / CO production rate (mol s−1) vs. reaction time (s).

[0089] The Faradaic efficiency (FEi) can be calculated by equations as follows: ^^^^^^^^ FE^ൌ ^^ൈ 100 %

[0090] where n0is initial moles of reactant; n is the moles of reactant after electrolysis; niis the moles of product i; zi is the number of electrons transferred for one product molecule; F is the Faraday constant (96,485 C mol−1); Q is the total charge passed through the electrolytic cell; t is the electrolysis time (s).

[0091] Materials characterization

[0092] Field-Emission Scanning Electron Microscope (FE-SEM) was recorded on the FEI Teneo LoVac. Aberration-corrected HAADF STEM imaging was performed using a probe-corrected Thermo Fisher (FEI) Titan Themis. Powder X-ray diffraction (XRD) patterns of samples were recorded on a Bruker X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA) over the range of 5 to 80 of 2θ degrees. X-ray photoelectron spectrometry (XPS) was recorded on a PerkinElmer PHI ESCA system with Physical Electronics (PHI). Nitrogen sorption isotherms were measured by the Micromeritics 3Flex analyzer at -196 °C. Before recording the N2sorption isotherms, the samples were pretreated at 200 °C under vacuum for 12 hours. The total surface area was calculated using the Brunauer–Emmett–Teller (BET) equation. The micropore and mesopore surface area, and micropore volume were evaluated using t-plot method. The mesopore volume and pore size distribution was determined by Barrett-Joyner-Hallenda (BJH) method. Inductively coupled plasma optical emission spectroscopy (ICP-OES) for nickel loadings was performed using an Agilent 5800 spectrometer.

[0093] Computational Methods

[0094] The calculation of CO2adsorption energy to the Ni-SAC surface

[0095] The calculations are performed at the level of density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP). The interactions between valence electrons and ions were described by the projector augmented wave (PAW) method.Atty. Dkt. No.00100-0360-PCT The electron exchange and correlation energy were coped with the gradient corrected Perdew-Burke-Ernzerh of (GGA-PBE) functional. The kinetic cutoff energy for plane-wave basis set was set as 520 eV. The dispersion interactions were included using Grimme’s D3 correction. A k-point sampling of 5×5×1 with Monkhorst-pack scheme was used for integration over the Brillouin zone in reciprocal space and spin polarisation was turned on for all simulations. The convergency criteria for the electronic and ionic loops were set at 10−5eV and 0.02 eV Å−1, respectively.

[0096] The graphene unit cell dimensions were optimised to be a = b = 2.46 Å and γ = 120° with 20 Å distance between layers perpendicular to the surface. Then, a 4×4 supercell of graphene was used as the 4N-Gr support. A C-divacancy was created in the cell, and four C atoms were placed with N atoms to build the 4N-Gr support. The Ni atom was embedded into the coordination site to result in the Ni@4N-Gr structure. The atomic coordinates were fully relaxed. Ni metal atom remained in the same plane as the C atoms.

[0097] The adsorption energy Eadsof an adsorbed species (i.e., CO2or H2O) on the Ni@4N-Gr was calculated as: ^^ୟ^^ ൌ ^^୫୭୪ା ^୧ିୗ^େ െ ^^^୫୭୪ ^ ^^^୧ିୗ^େ^

[0098] where ^^୫୭୪ା ^୧ିୗ^େ, ^^^୧ିୗ^େ, and ^^୫୭୪are the energies of the catalysis with the molecule adsorbed, of the bare catalyst, and of the molecule, respectively.

[0099] The calculation of CO2in-pore concentration and local number distribution in nanopores

[0100] In the representative modelling systems in the MD simulations, a nanoslit formed by two parallel catalyst sheets was assumed to be rigid and contacts with solution reservoirs at two sides. The slit walls were measured laterally and the width of reservoir was around 12 nm. Periodic boundary conditions were applied in three directions. Different pore environments were studied, including the slit sizes (defined as the distance between the atomic centres of neighboured graphene sheets) of 10.0 nm, 2.7 nm, 1.5 nm, and 1.0 nm, respectively. The solution was composed of CO2 and water. The atom valence was determined via the DDEC6 analysis via the DFT calculations. To account for the CO2adsorption capability towards to catalyst, the force field parameters were optimized via GA algorithm.Atty. Dkt. No.00100-0360-PCT

[0101] To determine the thermal equilibrium number of water molecules in the system, MD simulations were run under an NVT ensemble and the number of water molecules in system was adjusted via monitoring the water density in a region that belonged to reservoir and was 2 nm away from the slit edges. Provided that the water number density in this region was steady for ~10 ns, the system was considered to be in the equilibrium condition. The determination of CO2amounts was done similarly to that of water molecules. To ensure the CO2 numbers in the nanoslit reached the equilibrium state, each slit case was calculated by more than 80 ns. Moreover, to further check the in-pore concentration variations, the CO2in- pore amounts and positions were calculated at different time segments (each around 20 ns).

[0102] Results and Discussion

[0103] Direct air capture (DAC) followed by electrochemical conversion, lowers the carbon intensity of fuels and chemicals. Typically, an alkaline solution captures CO2as K2CO3, and concentrated CO2 is released through energy-intensive drying and calcination steps at ~900 °C, requiring 8-10 GJ / tonne CO2. In carbon capture and utilization (CCU) processes, producing carbon monoxide (CO) from captured CO2 adds 13-16 GJ / tonne CO, considerably higher than the lower heating value (LHV) of ~10 GJ / tonne for CO.

[0104] Reactive capture integrates CO2capture and conversion into a single system. Electrochemical CO generation from atmospheric CO2, assisted by green hydrogen, enables the synthesis of carbon-neutral chemical feedstocks via CO hydrogenation and Fischer- Tropsch process. Bipolar membrane (BPM) and cation exchange membrane (CEM)-based electrolyzers have been used for carbonate electrolysis, with BPM systems showing greater feasibility and reliability by avoiding co-ion transport and neutralization and by eliminating the need for noble metal catalysts at the anode.

[0105] In the process of using KOH as the DAC medium, carbonate species predominate over bicarbonate in the post-capture solution: [CO32−(aq)] >> [HCO3−(aq)]. Carbonate electrolysis in BPM electrolyzers relies on the in-situ generation of CO2(i-CO2) by combining carbonate with H3O+from water dissociation in the BPM junction. A quantitative analysis shows that at operation current density of 100 mA / cm2, the theoretical i-CO2supply rate is ~ 0.7 mL min–1cm–2. Considering the basic bulk (pH ~12) and local (pH > 13) environments for i-CO2recapture to (Bi)carbonate, specifically at high current density >100 mA / cm2, the i-CO2 supply rate rapidly declines to < 0.7 mL min–1cm–2, creating more challenges for CO generation in this i-CO2-starved environment.Atty. Dkt. No.00100-0360-PCT

[0106] In the liquid-feed reactive capture systems, non-porous metal catalysts (e.g., Ag, Cu) have been used. These metal nanoparticles are typically deposited onto hydrophilic and conductive carbon paper substrates. However, without a transport domain to facilitate mass transport, the non-porous nature of these metal catalysts often leads to rapid performance degradation, particularly at high current densities > 200 mA / cm². Alternative approaches have utilized Ni single-atom or molecular catalysts, enabling i-CO₂-to-CO conversion on active metal single sites. Nonetheless, poorly designed support materials can obstruct i-CO₂ transport, while insufficient dispersion or aggregation of active sites on the support under bias further reduces their effective utilization. These issues especially pronounced in carbonate systems, where ultra-low i-CO₂ conditions exacerbate the performance drop.

[0107] Local environment in gas-CO₂ and carbonate electrolysis systems

[0108] In the gas CO2electrolyzer, concentrated CO2is fed into the back side of the cathode layers. CO2 transport and conversion occur in three layers (FIG.6C). The CO2 transport occurs in the porous transport layers (PTL), which generally consists of a hydrophobic gas diffusion layer (GDL) made from macropore-rich carbon-based materials and a microporous layer (MPL) to minimize mass transport resistance. Typically, the MPL, made up of porous materials with relatively small pore diameters (<50 nm), is crucial for promoting even CO2distribution across the catalyst surface, enhancing contact between CO2and active sites, and improving CO2 retention and accumulation at the catalyst interface.

[0109] In the carbonate system (FIG.6A), CO32−ions first diffuse from the bulk solution through the CL and reach the BPM / catholyte interface, where they generate reactive species, i-CO2. The i-CO2 then diffuses back to the CL for i-CO2-to-CO conversion. This process requires a hydrophilic environment to enable the transport of ions and molecules, but it increases the barrier for i-CO2 diffusion, making its transport and conversion more challenging.

[0110] Design of a permeable porous diffusion layer with anchored Ni single sites (Ni-PPDL)

[0111] In the carbonate system, this Example described the integration of the PTL, which benefits CO2retention and accumulation, and the CL, which facilitates efficient CO2binding and activation, into a single, unified conductive layer, namely via a permeable porous diffusion layer with anchored Ni single atoms (Ni-PPDL) (FIG.6B). The Ni-PPDL is implemented by airbrushing porous Ni single-atom catalysts that are prepared through aAtty. Dkt. No.00100-0360-PCT template-controlled coordination-condensation-carbonization synthesis. Ni2+is first coordinated with an amine precursor and polymerized with carbon tetrachlorides within the pores of silica templates, followed by removal of templates. This synthesis process enables the pre-coordination of Ni with N, improves Ni dispersion, and allows precise control over the morphology and porous structure.

[0112] The anchoring and dispersing Ni single sites on a porous nitrogen assembly carbon (Ni-NAC) matrix promotes contact between reactant and active sites. The Ni-PPDL comprises both the transport domain, formed by carbon-based porous diffusion channels, and a kinetic domain, containing active Ni single sites for CO₂-to-CO conversion. Thus, this Ni- PPDL in a carbonate electrolyzer integrates the functions of both the PTL and CL in a gas- phase CO₂ system.

[0113] The pore structures of the Ni-PPDL are controlled based on diffusion mode (pore channel distribution) and pore diameters. By employing three templates—SBA-15, KIT-6, and MCM-48—and post-synthesis NH3treatment, the pore structures are controlled to create a unidirectional diffusion mode with predominantly mesopores (pore diameter: 2-50 nm) (Uni-MP), a multidirectional diffusion mode with predominantly mesopores (Mul-MP), and a multidirectional diffusion mode with a high proportion of micropores (pore diameter: <2 nm) (Mul-MMP), respectively. A Ni-PPDL thickness of 40 µm and a contact angle of 95° were found to offer the greatest improvement in carbonate reduction performance.

[0114] The detailed structures of the catalysts in Ni-PPDL for all three samples—Uni- MP, Mul-MP, and Mul-MMP—were examined using XRD, Raman spectroscopy and HRTEM, which indicated similar graphitic carbon. XPS showed similar electronic properties, with N 1s spectra revealing four types of nitrogen (pyridinic, pyrrolic, graphitic, and oxidized N) in similar ratios. The Ni 2p spectra, with binding energies around ~854.2 eV, indicated slight oxidation of Ni. STEM-HAADF imaging confirmed the formation of single-atom Ni, while STEM-energy-dispersive X-ray spectroscopy (STEM-EDS) mapping suggested a uniform distribution of Ni on the support.

[0115] N2adsorption-desorption isotherms displayed type IV hysteresis loops, indicating a mesoporous structure. Brunauer–Emmett–Teller (BET) surface area and average pore size analysis indicated a predominance of mesopores in Uni-MP and Mul-MP, whereas Mul- MMP exhibited an enhanced proportion of micropores (FIGS.7A-7B). SEM, TEM, and HAADF-STEM imaging revealed morphological differences: Uni-MP displayed a cylindricalAtty. Dkt. No.00100-0360-PCT structure with unidirectional channels and a linear pore array, whereas Mul-MP and Mul- MMP exhibited nanosheet or nanosphere structures.

[0116] Carbonate electrolysis performance under various transport modes

[0117] Uni-MP was first incorporated into the BPM-based carbonate electrolysis system. By optimizing the Ni precursor amount, calcination temperature, and adjusting the pore channel length to be within the 0.35–0.6 μm range, a peak FECO of 40% at 100 mA / cm2was achieved. However, this FECOrapidly declined to below 30% at 200–300 mA / cm2(FIG.7C). The two-dimensional diffusion mode within the cylindrical pores restricted i-CO2 access and availability, thereby limiting CO selectivity at higher currents.

[0118] Switching to a multidirectional diffusion mode—featuring interconnected channels that enable three-dimensional diffusion—a slight increase in FECO on Mul-MP at the high current density range of 200–300 mA / cm2was observed, though FECOremained below 35% (FIG.7C). Importantly, with the transition to a multidirectional diffusion mode with significantly increased micropore density (Mul-MMP), FECO rose to 50%±3% as the micropore ratio increased from 7% in Mul-MP to 28% in Mul-MMP.

[0119] ICP-MS analysis confirmed that the increased catalyst activity in Mul-MMP was not primarily due to higher Ni content, as Mul-MMP exhibited the lowest Ni mass loading (1.35 wt%) among all samples. The normalized Turnover Frequency (TOF) for CO generation on Mul-MMP was 1.9 and 3.3 times higher than that of Uni-MP and Mul-MP, respectively, at 300 mA / cm2. These results highlight that improved mass transport of i-CO2 and efficient utilization of active sites, rather than the number of active sites in the Ni-PPDL, are the primary factors contributing to the high activity.

[0120] Characterization of i-CO2 transport kinetics in pores

[0121] Electrochemical impedance spectroscopy (EIS) was conducted and the electric double-layer capacitance (Cdl), which represents the electrochemically accessible surface area (electrolyte-wetted) for the reaction, was calculated. The BET area provides the physical surface area of the catalyst. Normalizing Cdlto BET (i.e., Cdl / BET) (FIG.7D) reflects the fraction of the catalyst's physical surface area that actively participates in the electrochemical reaction, indicating the active site utilization. In a 1.5 M K2CO3electrolyte, the total capacitance of Mul-MMP (576 F g⁻¹) exceeds that of the other two catalysts (427 F g⁻¹ for Uni-MP and 487 F g⁻¹ for Mul-MP). Notably, after normalization, Mul-MMP achieved the highest Cdl / BET of 0.64.Atty. Dkt. No.00100-0360-PCT

[0122] Next, CO2 diffusion and affinity were examined under dry, wetted, and wetted with applied bias conditions. Under dry conditions, CO2affinity was assessed using CO2adsorption isotherm measurements. Mul-MMP showed increased CO2 uptake over a wide pressure range. The measured CO2adsorption heat for Mul-MMP ranged from 24 to 36 kJ mol⁻¹, consistently higher than that of Uni-MP and Mul-MP.

[0123] Under wet conditions, reactant retention within the pores was evaluated, as determined by the relaxation time constant (τo) (FIG.7E), measured via EIS. τoreflects the rate capability of reactant transport in the pores, essentially representing the penetration resistance, which is sensitive to both the pore size and the pore tortuosity. FIG.7E shows a trend in τo: Mul-MMP (20 ms) > Mul-MP (16 ms) > Uni-MP (10 ms). In more diluted electrolytes, the τ₀ differences among the catalysts further increased (FIG.7E).

[0124] Under biased conditions, i-CO2transport and retention through was assessed by analysis in gas CO2 electrolysis system. In three-compartment flow cells, Mul-MMP maintained a high FE of > 90% even as CO2flow rate reduced to 2.5 mL min⁻¹ cm⁻² and CO2partial pressure dropped to 20%. In a membrane electrode assembly (MEA)-based CO2 electrolyzer with an anion exchange membrane (AEM), Mul-MMP achieved an impressive single-pass (SP) efficiency of 45% (FIG.7F) at 300 mA cm⁻2.

[0125] The experimental results above indicate that the pore structure of Mul-MMP in Ni-PPDL enhances active site utilization and CO2affinity, meanwhile also extending i-CO2retention time within the pore. This enables deeper i-CO2 penetration into the pores, leading to a greater accumulation of i-CO2within the nanoconfined space.

[0126] Simulation of i-CO2transport kinetics in pores

[0127] Density function theory (DFT) calculations were performed to evaluate the CO2 adsorption energy as a function of the normal distance from the catalyst surface. When the surface is uncharged, and as the distance decreases from 9 Å to 3 Å, the CO2 adsorption energy changed from zero (non-adsorption) to negative values (adsorption) and eventually to positive values (repulsion). A maximum adsorption energy of −0.175 eV was obtained at a distance of 3.1 Å, where the O-C-O angle remains at 180° and aligns above the Ni atom, indicating a physisorption interaction between CO2and uncharged Ni-NAC. Removing Ni atoms from the model results in lower CO2 adsorption energy on pure graphene, indicating that the atomically dispersed Ni enhances CO2physisorption. On a charged Ni-NAC surface, CO2 adsorption is further enhanced, allowing CO2 to chemisorb to the surface. This isAtty. Dkt. No.00100-0360-PCT evidenced by an upward shift of the d-band center when Ni is atomically isolated, which reduces electron density in the antibonding state and stabilizes CO2. It was also shown that the H2O adsorption energy on Ni-NAC is less than half that of CO, indicating that H2O reduction to H2is less favored.

[0128] The force field molecular dynamics (MD) was further conducted to investigate CO2 distribution in nanoconfined pore environments filled with water molecules. The MD model features a nanoslit in contact with bulk reservoirs on both sides, reflecting the porous structure of the catalysts. Three slit sizes were considered, 1.0 nm, 1.5 nm, and 10 nm, along with various CO2concentrations in the bulk reservoir to represent the dynamic CO2concentrations in reactive capture systems operating at different current densities. The force field parameters in the MD simulations were refined using an in-house genetic algorithm, training them to closely match the DFT results, with a primary focus on uncharged cases.

[0129] The results showed that reducing the slit size to sub-2 nm significantly enhances CO2participation within the pore. When the slit size is 10 nm, the in-pore CO2concentration is equal to the bulk reservoir concentration. As the slit size decreases to below 2.0 nm, the in- pore concentration largely surpasses the bulk concentration by up to 4 times. This enhanced participation is a result of the enriched CO2 adsorption in the near-surface region, i.e., two sharp CO2density peaks near the surface, which is consistent with the DFT results. As pore size decreases, the pore volume occupied by these CO2 layers increases, raising the overall CO2in-pore concentration.

[0130] The nanoconfinement effect is attributed to the short-range non-electrostatic interactions between the pore surface and CO2 molecules, which leads to a significant increase in the reorientation and rearrangement of CO2within the pore. MD results also confirm that H2O does not interfere with CO2 adsorption at the catalyst surface, consistent with its weaker surface adsorption energy.

[0131] The mobility / transport kinetics of CO2within confined pores was analyzed through its survival time correlation function, S(τ), which denotes the likelihood of CO2 remaining within a specified region over a time interval τ. The S(τ) for CO2in the interfacial (within 5.0 Å distance from the surface) and central regions of the nanoslit was analyzed. The ^^^^^^ of interfacial region decays slower than the central region, suggesting a lower CO2mobility at the interface. In addition, the smaller pores have a higher interfacial ^^^^^^ value, indicating that the size confinement further suppresses the transport dynamics of CO2.Atty. Dkt. No.00100-0360-PCT

[0132] Optimizing reactive capture system to increase energy efficiency

[0133] Mul-MMP was employed as the Ni-PPTL in the BPM electrolyzer. It was observed that FECO was maintained >40% at 100-400 mA / cm2(FIG.8A-8B), with a peak FECO of 50%±3% observed at 300 mA / cm2. The FECO outperformed the best-prior reported results in the BPM-based carbonate electrolyzer.

[0134] In addition to the CO selectivity, energy efficiency (EE) is also largely dependent on the full cell voltages. Sensitivity analysis revealed that, under a baseline scenario with FECO of 30% and a Vcell of 3.4 V, the estimated energy consumption is 64 GJ / tonne of syngas. It was found that both FECO and Vcell significantly affect energy consumption; variations of ±20% in FECOand ±0.5 V in Vcelllead to changes in energy consumption of +8 to -11 GJ for 1 tonne of syngas production. This projected energy consumption assumes the use of an efficient water electrolyzer to supply the "missing" H2or a solid oxide electrolyzer cell (SOEC) to provide the "missing" CO toward the final syngas composition of 2:1 H2: CO.

[0135] The carbonate electrolyzer was further optimized for a higher EE. At 300 mA / cm2, a Vcellof 5.2 V was recorded (FIG.8C). Using a diagnostic electrolyzer equipped with two reference electrodes, the BPM was found to account for 53% of the total Vcell. The BPM was replaced with a configuration that included a nanoparticle SnO₂ WD catalyst and a NiFeP anode, resulting in a reduced Vcell of 3.3 V at 300 mA / cm2. Ultimately, a syngas EE of 46% at this current density was achieved (FIG.8C), surpassing the previous best EE of 28% under the same conditions.

[0136] As an initial check of the operating stability of the full electrolyzer with Mul- MMP cathode, SnO2-BPM, and NiFeP anode, the system was characterized at 200 mA / cm2for 50 hours of electrolysis (FIG.8D). The system retained FE >40% and Vcell of ~3.0 V with periodically replacement of the hydrophilic mixed cellulose ester (MCE) interposer layer. X- ray absorption near-edge structure (XANES) spectra confirmed the similarity in the Ni K edges before and after electrolysis. X-ray absorption fine structure spectrometry (EXAFS) showed the local coordination environment of Ni atoms, with a dominant peak at around 1.4 Å, assignable to the Ni-N coordination, confirming the stable Ni single-atom structure. This stable Ni-N coordination anchors Ni atoms within the carbon-nitrogen framework, preventing the aggregation or migration of Ni under biased conditions. The anchored Ni within the porous matrix also prevented the formation of Ni particles that could block the micropore channels and resist diffusion of i-CO2.Atty. Dkt. No.00100-0360-PCT

[0137] The carbon utilization was defined as follows: ^^ Carbon utilization ൌ ^1 െ ^ைଶ ^^^^^^ ^ ൈ 100 %^ைଶ

[0138] where ^^^^ைଶis the number of moles of CO2 can be generated from the BPM-based electrolyzer; ^^^ைଶ ^^^is the number of moles of gas CO2 detected at the outlet of the catholyte. We observed carbon utilization of >99% at 100-400 mA / cm2, with CO2 concentration incathode outlet <1%. The outlet CO concentration reached ~50% at 200- 300 mA / cm2. Such carbon utilization and CO concentration are the highest, looking across both reactive capture and gas-fed CO2reduction systems reported to date.

[0139] An energy consumption analysis of various systems was conducted: i) sequential capture-and-release followed by gas-fed electrochemical CO2 upgrade; ii) sequential capture- and-release followed by reverse water-gas shift (RWGS) using hydrogen from an efficient water electrolyzer; iii) the integrated reactive capture approach explored in this Example, with H2is supplemented from water electrolyzer to obtain H2:CO of 2:1 syngas. The integrated reactive capture process offers the lowest projected energy expense in GJ / tonne of syngas (43 GJ / tonne), its advantage deriving the avoidance of the CO2 regeneration and circulation steps in sequential DAC-plus-electrolysis.

[0140] Scale-up of Ni-PPTL for carbonate to syngas

[0141] Both the electrode and MEA were scaled up, testing a 25 cm² surface area electrode for syngas production via carbonate electrolysis (FIG.9A). At currents of 1.25 A and 2.5 A, the FECO on the Ni-PPTL electrode was measured at 38.4% and 33.1%, respectively (FIG.9B). It was further observed that the CO2concentration in the cathode outlet remained < 1%, indicating the production of nearly pure syngas downstream, with CO concentrations of 38.1% at 1.25 A (FIG.9B). This significantly minimized CO2circulation and separation challenges. This syngas composition approached a H2:CO ratio of 2:1, making it suitable for potential direct downstream utilization of pure syngas.

[0142] Additionally, the time-average CO production rate was recorded, which stabilized at 8.6 and 16.0 mmole h-1, respectively, over 5 hours of operation (FIG.9C). This corresponds to the removal of 1.9 and 3.4 grams of CO2from the carbonate capture liquid (FIG.9D), which is subsequently converted into CO. These results enable the commercialization of the carbonate system in DAC-to-product conversion.Atty. Dkt. No.00100-0360-PCT

[0143] Conclusion

[0144] In this Example, a Ni-PPTL was designed that integrates i-CO2 transport and conversion into a single, uniform layer. This Ni-PPTL consists of single-atom Ni anchored in the carbon-nitrogen matrix with multidirectional porous channels. Benefiting from the short- range non-electrostatic interactions between i-CO2and the pore walls, i-CO2accumulates and concentrates within the micropores. In a BPM-based electrolyzer for carbonate electrolysis, the implementation of such a porous layer achieved a record EE of 46% at 300 mA / cm².

[0145] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”

[0146] If not already included, all numeric values of parameters in the present disclosure are proceeded by the term “about” which means approximately. This encompasses those variations inherent to the measurement of the relevant parameter as understood by those of ordinary skill in the art. This also encompasses the exact value of the disclosed numeric value and values that round to the disclosed numeric value.

[0147] The foregoing description of illustrative embodiments of the disclosure has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described to explain the principles of the disclosure and; as practical applications of the disclosure, to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto and their equivalents.

[0148] In recognition of the inherent nature of electrochemical processes, throughout the present disclosure, terms and phrases such as “absence,” “free,” “does not comprise,” “does not occur,” etc. encompass, but do not require a perfect absence of the referenced entity.

[0149] The term “type” as used herein refers to chemical formula such that a single type means the same chemical formula and different type means different chemical formula. Similarly, use of “more” as in “one or more” refers to use of different types of the relevantAtty. Dkt. No.00100-0360-PCT entity. Terms such as “comprising” and the like may be replaced with terms such as “consisting” and the like.

Claims

Atty. Dkt. No.00100-0360-PCT WHAT IS CLAIMED IS:

1. An electrolyzer comprising: a cathode comprising a single metal atom catalyst comprising single metal atoms distributed within a mesoporous carbon matrix, the cathode in contact with a catholyte comprising carbonate ions; an anode in electrical communication with the cathode; and a bipolar membrane configured to generate in situ CO2in the catholyte.

2. The electrolyzer of claim 1, wherein the mesoporous carbon matrix is a mesoporous carbon nitride matrix and single metal atom catalyst is a metal-nitrogen assembly carbon (M-NAC) catalyst.

3. The electrolyzer of claim 2, wherein M is selected from Ni, Fe, Co, Cu, Pt, Pd, Ru, Rh, and a combination thereof.

4. The electrolyzer of claim 2, wherein M is Ni.

5. The electrolyzer of claim 1, wherein the single metal atom catalyst is doped with a dopant element.

6. The electrolyzer of claim 5, wherein the dopant element is a non-carbon, non- metal element.

7. The electrolyzer of claim 5, wherein the dopant element is selected from S, B, P, and a combination thereof.

8. The electrolyzer of claim 5, wherein the single metal atom catalyst is a P- doped M-NAC catalyst.

9. The electrolyzer of claim 8, wherein M is selected from Ni, Fe, Co, Cu, Pt, Pd, Ru, Rh, and a combination thereof.

10. The electrolyzer of claim 8, wherein M is Ni.

11. The electrolyzer of claim 1, wherein the catholyte comprises an aqueous alkali metal carbonate solution.Atty. Dkt. No.00100-0360-PCT 12. The electrolyzer of claim 1, further comprising an interposer layer between the cathode and the bipolar membrane, the interposer layer configured to suppress hydrogen evolution.

13. The electrolyzer of claim 4, wherein the single metal atom catalyst comprises micropores and mesopores.

14. An electrolyzer comprising: a cathode comprising a Ni-NAC catalyst comprising single nickel atoms distributed within a mesoporous carbon nitride matrix, the cathode in contact with an aqueous alkali metal carbonate catholyte comprising carbonate ions; an anode in electrical communication with the cathode; a bipolar membrane configured to generate in situ CO2in the catholyte; and an interposer layer between the cathode and the bipolar membrane.

15. The electrolyzer of claim 14, wherein the Ni-NAC catalyst further comprises phosphorous doped therein.

16. A method for producing syngas, the method comprising generating the in situ CO2in the catholyte of the electrolyzer of claim 1 and reducing the in situ CO2to syngas at the cathode.

17. A doped single metal atom catalyst comprising a dopant element and single metal atoms distributed within a mesoporous carbon matrix.

18. The doped single metal atom catalyst of claim 17, wherein the dopant element is a non-carbon, non-metal element.

19. The doped single metal atom catalyst of claim 17, wherein the dopant element is selected from P, S, B, and a combination thereof.

20. The doped single metal atom catalyst of claim 19, wherein the single metal atoms comprise Ni, the mesoporous carbon matrix is a mesoporous carbon nitride matrix, and the dopant element is P.

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