A membrane-free co-production process for formate via carbon dioxide electroreduction coupled with alcohol oxidation.

Abstract

This invention relates to a membrane-free process for the co-production of formate via carbon dioxide electroreduction coupled with alcohol oxidation, belonging to the field of electrocatalysis. Based on the coupling of cathode carbon dioxide electroreduction and anodic small-molecule alcohol oxidation, this invention establishes a novel ion-exchange membrane-free process for the co-production of formate at both cathode and anode. Compared to traditional carbon dioxide electroreduction processes, this invention fundamentally eliminates the mass transfer resistance of ions at the exchange membrane, avoiding secondary carbon emissions during CO2 electroreduction. Simultaneously, the co-production of formate at both cathode and anode simplifies the product separation process and reduces separation costs from a product engineering perspective. By avoiding the use of expensive and periodically replaceable ion exchange membranes, the process operating costs are reduced, while the equipment's operating cycle is extended. The proposed replacement of the OER reaction with small-molecule alcohol oxidation significantly reduces process energy consumption and yields formate products with a much higher added value than oxygen, providing a novel approach for the industrial application of carbon dioxide electroreduction.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalysis and fine chemical synthesis, specifically relating to a membrane-free process for the co-production of formates (such as potassium formate, sodium formate, etc.) by carbon dioxide electroreduction coupled with alcohol oxidation. Background Technology

[0002] With economic and social development, human activities have led to a dramatic increase in the use of fossil fuels, resulting in massive emissions of carbon dioxide (CO2) and a severe environmental crisis. As a major component of greenhouse gases, controlling CO2 emissions and achieving its comprehensive utilization has become a significant challenge for sustainable social development. Utilizing electrocatalysis to convert CO2 into high-value chemicals is a crucial technology for constructing an artificial closed carbon cycle through atomistic economics. This technology can be carried out at ambient temperature and pressure, producing diverse products that can be targeted and controlled according to demand, generating profound social and economic benefits in today's low-carbon context.

[0003] Current research on electrocatalytic CO2 reduction technology mainly focuses on catalyst design and the exploration of catalytic reaction mechanisms. Carbon monoxide, as the most common product of CO2 reduction, has achieved a Faradaic efficiency exceeding 95%. In particular, Au, Ag, and transition metal single-atom catalysts have demonstrated excellent catalytic performance. Metal-based catalysts, represented by Bi, Sn, and In, have also achieved Faradaic efficiencies exceeding 90% for formic acid, and their current densities meet industrial requirements. Copper-based materials are among the few catalysts capable of producing multi-carbon products; however, due to the complexity of multi-electron reaction pathways and the diversity of products, it remains difficult to obtain high-purity single multi-carbon products. Furthermore, the subsequent complex product separation process generates secondary carbon emissions, thus limiting their practical application. From an economic perspective, formic acid and carbon monoxide are currently the most promising products for the electrocatalytic reduction of carbon dioxide.

[0004] Currently, research on CO2 reduction mainly focuses on catalyst design. A few researchers also explore improving CO2 reduction performance through electrolyzer design and solid electrolyte development. As research deepens, industrial current density that meets industrial needs has become a crucial indicator. Traditional H-type electrolyzers, due to their low CO2 solubility and diffusion rate, severely limit the CO2 concentration required for high electron transfer. Flow-through electrolyzers or membrane electrode electrolyzers with gas diffusion electrodes have opened new avenues for the industrialization of electrocatalytic CO2 reduction. A gas-liquid-solid three-phase interface can be formed on the cathode catalyst surface, significantly increasing the CO2 concentration around the active sites and eliminating CO2 diffusion limitations.

[0005] In current CO2 reduction research, the anodic reaction is mainly the oxygen evolution reaction (OER). Anodic OER reactions account for over 80% of the theoretical energy consumption of the entire electrolyzer, and the generated O2 has low economic value, typically being directly emitted into the atmosphere, resulting in resource waste and low energy utilization efficiency. Developing novel alternative anodic reactions has become a new research hotspot. Studies and inventions have reported that small-molecule organic compounds such as methanol, ethanol, 5-hydroxymethylfurfural, and urea can serve as substrates to replace OER reactions. In comparison, the required electrode voltage is lower than that of OER reactions, and it can realize the conversion of low-value-added compounds into high-value-added fine chemicals, further reducing the cost of electrocatalytic CO2 reduction and improving economic efficiency. However, the products of most anodic oxidation reactions differ from those of cathode CO2 reduction, inevitably requiring ion exchange membranes to separate the anode and cathode chambers to avoid subsequent complex product separation processes and corresponding energy costs caused by cross-contamination of anode and cathode reaction products. However, the use of ion exchange membranes brings several challenges to CO2 reduction: Firstly, the presence of ion exchange membranes significantly increases the energy consumption of large-scale industrial CO2 reduction processes, due to industrial current conditions (>100mA cm⁻¹). -2 Firstly, the presence of ion exchange membranes significantly increases the mass transfer resistance of ions, causing a sharp increase in the overpotential of electrocatalytic CO2 reduction and resulting in secondary carbon emissions during CO2 electroreduction. Secondly, alcohol reagents have significant incompatibility issues with current ion exchange membranes, which can cause ion exchange membrane swelling and lead to rapid failure of the ion exchange membrane. Thirdly, ion exchange membranes are consumables in the CO2 electroreduction industrial process and have high costs; regular replacement of ion exchange membranes will significantly increase the operating costs of the process. Summary of the Invention

[0006] This invention addresses the problems of ion mass transfer obstruction, rapid membrane failure, and high operating costs caused by the use of ion exchange membranes in traditional CO2 electroreduction processes. Based on the coupling of anode small molecule alcohol (e.g., methanol, ethylene glycol, glycerol) oxidation and cathode CO2 electroreduction, a novel ion-exchange membrane-free process for the co-production of formate at both the anode and cathode is established. To overcome the shortcomings of existing CO2 electroreduction technologies, this invention aims to provide a membrane-free process for the co-production of formate by coupling CO2 electroreduction with alcohol oxidation. This process fundamentally eliminates the mass transfer resistance of ions at the exchange membrane, minimizing the operating cost of the CO2 electroreduction industrial process from the perspective of enhanced ion mass transfer, and avoiding secondary carbon emissions caused by voltage planning in the CO2 electroreduction industrial process. The process design for the co-production of formate at both the anode and cathode greatly simplifies the product separation process from a product engineering perspective, while obtaining high-value-added formate fine chemical products. From a process economic perspective, the ion-exchange membrane-free process significantly reduces the operating costs associated with periodic ion exchange membrane replacement, ensuring stable long-term operation of the equipment.

[0007] In order to achieve the above-mentioned objectives and solve the problems existing in the prior art, the technical solution adopted by the present invention is: a process method for co-producing formate by carbon dioxide electroreduction coupled with alcohol oxidation without a diaphragm, which uses a flow-through electrolytic cell as an electrolysis device and utilizes cathode carbon dioxide reduction coupled with anodic alcohol oxidation to achieve co-production of formate;

[0008] The circulating electrolytic cell includes a gas chamber, a cathode chamber, and an anode chamber. A gas diffusion electrode is used to separate the gas chamber and the cathode chamber. The cathode electrolyte and the anode electrolyte are the same alcohol-containing potassium hydroxide or sodium hydroxide electrolyte. The alcohol in the electrolyte is methanol, ethanol, ethylene glycol, or glycerol.

[0009] The catalyst for cathodic reduction is selected from metal nanoarrays, alloys, metal oxides, sulfides, organometallic frameworks, and carbon-coated metal nanomaterials containing bismuth, tin, antimony, and indium; the selectivity of the cathodic reduction catalyst for formic acid is greater than 80%, and the Faraday efficiency for formic acid is greater than 80%.

[0010] The catalysts for anodic oxidation are iron, cobalt, nickel, and copper transition metal oxides and hybrids and alloys. The catalysts have a Faradaic efficiency of more than 80% for formate and formate is the only product of small molecule alcohol oxidation.

[0011] The cathode catalyst is selected from Bi nanosheets, BiSn alloy nanospheres, SnS / aminocarbon, In-MOF, Bi-MOF, and BiSn aerogel; the anode catalyst is a nickel-based nanomaterial selected from nickel hydroxide, nickel sulfide, nickel nitride hybrids, nickel-molybdenum, nickel-cobalt alloy oxides, NiCo-MOF, NiCo-LDH, and Ni-Mo-N nanosheets.

[0012] The electrolysis process is as follows: renewable and clean energy sources such as solar and wind power are used as the direct power source for the electrolytic cell. The captured CO2 is introduced into the cathode gas chamber, and the electrolyte containing small molecule alcohols enters the anode and cathode chambers respectively. On the cathode side, CO2 gains electrons at the gas-liquid-solid three-phase interface through the gas diffusion electrode and undergoes a reduction reaction to obtain formic acid. On the anode side, the small molecule alcohols lose electrons and are oxidized to formic acid. The entire electrolysis process avoids the use of ion exchange membranes. The full cell voltage range is 2-5V. The electrolysis process is continuous, and unreacted carbon dioxide can be recycled.

[0013] Product separation section: For gaseous products, pressure swing adsorption is used to recover and recycle unreacted carbon dioxide. Liquid products need to be neutralized with formic acid and distilled under reduced pressure to obtain high-purity formate products.

[0014] For the gaseous products generated by cathodic electroreduction, when the formic acid Faraday efficiency is greater than 90%, carbon dioxide is recovered by pressure swing adsorption, while hydrogen and carbon monoxide products are collected as syngas without separation.

[0015] This scheme can be powered by renewable and clean energy sources such as solar and wind power. Renewable energy power generation can directly drive the ion-exchange membrane electrolysis process without the need for voltage boosting. This avoids the price increase and resistance consumption caused by the inverter processing and grid connection required for renewable energy. In the circulating electrolyzer, the reduction of carbon dioxide at the cathode coupled with the oxidation of alcohols (such as methanol, ethylene glycol, glycerol, etc.) at the anode can achieve double the production of formate (such as potassium formate, sodium formate, etc.) on an industrial scale.

[0016] Catalyst design is key to realizing the ion-exchange membrane electrolysis process. This process is universal and has relatively relaxed requirements for catalysts. It only needs to guide the formate products of cathodic reduction and anodic oxidation to have high selectivity and Faraday efficiency, so as to avoid the formation of other products that would increase the energy consumption and process complexity of the separation process.

[0017] Since the electrolytes and products at both the anode and cathode are the same, the device can operate stably without the use of ion exchange membranes, without causing cross-contamination of products that would lead to additional energy consumption in the separation process. This eliminates the mass transfer resistance of ions in the exchange membrane, eliminates secondary carbon emissions caused by corresponding voltage polarization, and avoids membrane swelling that would increase costs and shorten the device's operating cycle.

[0018] When the formic acid Faraday efficiency is greater than 90%, the gaseous products generated by the cathodic electroreduction only need to be recovered by pressure swing adsorption to recover carbon dioxide, while the hydrogen and carbon monoxide products can be sold directly as syngas without separation.

[0019] To meet the process current density requirements, a designable flow-through electrolyzer is introduced as the electrolysis equipment. The electrolyte can be a potassium hydroxide or sodium hydroxide solution containing different concentrations of small molecule alcohols (e.g., methanol, ethylene glycol, glycerol). Since the anode and cathode products are identical, and the presence of small molecule alcohols does not affect the performance of the cathode catalyst, the anode and cathode can share the same electrolyte, which is circulated within the flow-through cell using a pump. More importantly, because there is no product cross-contamination or impact on catalyst activity, the anode and cathode chambers do not require ion exchange membranes for separation, thus avoiding the use of expensive ion exchange membranes, significantly extending the unit's operating cycle, and reducing process operating costs.

[0020] This process can use renewable and clean energy sources such as solar and wind power as the power source. Renewable energy power generation can directly drive the ion exchange membrane electrolysis process without the need for voltage boosting. This avoids the price increase and electricity consumption caused by the inverter processing and grid connection transmission required for renewable energy. With the further development of clean energy, the cost of electricity is expected to decrease further.

[0021] For catalyst design, this process does not require strict design; it only needs to meet the following requirements to be applicable. First, the cathode catalyst must have a high Faradaic efficiency (greater than 80%) for formic acid. According to current research results, metals such as bismuth, tin, and indium, as well as their hybrids, alloys, metal-organic frameworks, and carbon-coated metal nanomaterials can all be used as cathode catalysts. Catalysts with inexpensive raw materials, simple synthesis processes, and scalable preparation are preferred.

[0022] For anode catalysts, nickel-based nanomaterials are mainly selected, such as nickel hydroxide, nickel sulfide, nickel nitride and other hybrids, as well as nickel-molybdenum, nickel-cobalt and other alloy oxides, which have excellent catalytic activity and selectivity for the oxidation of small molecule alcohols to formic acid. It should be noted that noble metals such as Pd and Pt have poor effects on the oxidation of glycerol in alkaline electrolytes and are prone to over-oxidation to produce CO and poison the catalyst. The selection of anode catalysts should meet two key conditions: (1) the anode catalyst has a high Faradaic efficiency (greater than 80%) for formic acid; (2) formic acid is the only product of the oxidation of small molecule alcohols. In addition, the overall design of anode catalysts should also follow the principles of low raw material cost, simple synthesis method and large-scale preparation.

[0023] Theoretically, the single-pass conversion rate of CO2 is only 30% for the separation of gaseous products. Unreacted CO2 can be recovered and recycled using pressure swing adsorption (PSA). Other separated gaseous products are generally a mixture of CO and H2, which can be separated by PSA after concentration. If the catalyst has good performance and the concentration of CO and H2 produced is low, they can be sold directly as syngas or used as thermal utilities in subsequent distillation processes.

[0024] Considering that the product concentration of each reaction is low, it is necessary to first perform cyclic electrolysis and concentration. After the formate in the electrolyte accumulates to a certain level, it is then neutralized with formic acid to adjust the pH to neutral, and further dehydrated by vacuum distillation to obtain a high-purity formate product.

[0025] The advantages of this invention are as follows: Compared with the traditional CO2 electroreduction process, the novel membrane-free process involved in this invention has four advantages: (1) It fundamentally eliminates the diffusion resistance of ions at the exchange membrane, greatly reduces voltage polarization under industrial high current density, and thus avoids secondary carbon emissions in the CO2 electroreduction process; (2) The cathode and anode co-produce formate, which simplifies the product separation process from the perspective of product engineering, reduces separation costs, and obtains double the formate product yield at one time; (3) This process is operated in a membrane-free device, thus avoiding the use of expensive and regularly replaced ion exchange membranes, thereby reducing process operating costs and extending the device operating cycle; (4) The invention proposes to replace the OER reaction with small molecule alcohol oxidation, which can drive the reaction under the same industrial current density with a lower electrode voltage, improves energy utilization efficiency, and compared with the lower value of oxygen, alcohol oxidation can realize the conversion of high value-added fine chemicals and obtain higher economic benefits.

[0026] The process proposed in this invention is universal and applicable to many catalyst systems with high activity towards formic acid, providing a new approach for the commercial development of electrocatalytic CO2 reduction and showing broad prospects. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the device in the embodiment.

[0028] Figure 2 These are the LSV curves with and without ion exchange membranes in the embodiments.

[0029] Figure 3 This is a comparison of LSVs of glycerol oxidation coupled with CO2 reduction and OER coupled with CO2 reduction in the examples.

[0030] Figure 4 This is the 1H NMR spectrum analysis of the reaction products in the examples.

[0031] Figure 5 These are liquid phase spectra before and after the reaction in the example. Detailed Implementation

[0032] The present invention will be further described below with reference to the embodiments.

[0033] It should be noted that this process does not require strict design for the catalyst; it only needs to meet the following requirements. First, the cathode catalyst must have a high Faradaic efficiency (greater than 80%) for formic acid. According to current research, metals such as bismuth, tin, and indium, as well as their hybrids, alloys, metal-organic frameworks, and carbon-coated metal nanomaterials, can all be used as cathode catalysts. Catalysts with inexpensive raw materials, simple synthesis processes, and scalable preparation are preferred. For the anode catalyst, nickel-based nanomaterials are mainly selected, such as nickel hydroxide, nickel sulfide, nickel nitride hybrids, and nickel-molybdenum and nickel-cobalt alloy oxides, which have excellent catalytic activity and selectivity for the oxidation of small molecule alcohols to formic acid. It is particularly important to note that noble metals such as Pd and Pt are less effective at oxidizing glycerol in alkaline electrolytes and are prone to over-oxidation, producing CO and poisoning the catalyst.

[0034] The selection of an anode catalyst must meet two key conditions: (1) the anode catalyst must have a high Faraday efficiency (greater than 80%) for formic acid; and (2) formic acid must be the only product of the oxidation of small molecule alcohols. Furthermore, the overall design of the anode catalyst should also adhere to principles such as low raw material cost, simple synthesis method, and scalable preparation. Therefore, the catalysts in the following examples are used as application examples and are not intended to limit the scope.

[0035] Example 1

[0036] Utilizing the redox capabilities of metals, aluminum foil was reacted in an aqueous solution containing bismuth salt. The precipitate was collected and dried to obtain a cathode catalyst with high selectivity for reducing CO2 to formic acid. For the anode catalyst, a one-step hydrothermal-etching method was used to synthesize NiCo-MOF catalyst. The specific method is as follows: Commercially available nickel foam was cleaned sequentially with HCl, ethanol, and water under ultrasonic conditions to remove the surface oxide layer. Simultaneously, 1 mmol Ni(NO3)3·6H2O and 0.5 mmol Co(NO3)3·6H2O were dissolved in 15 mL of methanol as solution A, and 4 mmol 2-methylimidazole was dispersed in 15 mL of methanol as solution B. After stirring for 20 minutes, solution B was rapidly added to solution A and stirring continued. Then, the mixed solution and the cleaned nickel foam were transferred to a polytetrafluoroethylene-lined stainless steel autoclave and subjected to a hydrothermal reaction at 65-80 °C for 12 h. Finally, after washing several times with deionized water and ethanol, the synthesized NiCo-MOF / NF catalyst was dried overnight at 60 °C.

[0037] 10 mg of synthesized Bi-based catalyst powder was weighed and ultrasonically dispersed in an ethanol solution containing 20% ​​Nafion to prepare a uniform catalyst ink, which was then uniformly drop-coated onto a gas diffusion electrode. A self-supporting NiCo-MOF / NF catalyst was used as the anode. An electrolyte containing 0.3 M glycerol and 1 M KOH was employed. Electrochemical performance tests were conducted using a Shanghai Chenhua CHI 760E workstation in a flow-through electrolyzer, verifying the feasibility of the process. Specifically, the gas diffusion electrode coated with the Bi-based catalyst was used as the working electrode, Ag / AgCl as the reference electrode, and commercially available nickel foam as the counter electrode. Polarization curves were tested. By comparing the polarization curves before and after the addition of glycerol to the electrolyte, it was determined that the introduction of glycerol did not affect the performance of the cathode catalyst. Furthermore, using a NiCo-MOF / NF catalyst as the anode, polarization curves were tested in a two-electrode system. The polarization curves were compared with and without glycerol in the electrolyte, and with and without an ion exchange membrane. It was found that compared to the OER reaction, glycerol oxidation coupled with CO2 reduction has a significant energy-saving effect. Moreover, removing the ion exchange membrane further reduces energy consumption, thanks to the elimination of interfacial resistance. The feasibility of this process was further verified through stability testing. The process was successfully tested at 100 mA cm⁻¹ for 20,000 s. -2 In the current density stability test, its voltage fluctuation is small.

[0038] at last Figure 4 Qualitative and quantitative analysis of the electrolysis products was performed using proton nuclear magnetic resonance spectroscopy. The results showed that after 10 hours of electrolysis, almost all of the reactant glycerol was converted into formate, and the liquid reduction product of carbon dioxide at the cathode was also only formate, which directly proved the feasibility of the process.

[0039] Example 2

[0040] Using bismuth nitrate and trimesic acid as precursors and methanol as solvent, a uniform rod-shaped Bi-MOF catalyst was prepared by reacting at 120°C for 24 hours via a solvothermal method, followed by centrifugation and drying. This catalyst was used as the cathode for carbon dioxide reduction and was drop-coated onto a gas diffusion electrode using the same method as in Example 1, serving as the cathode-side electrocatalyst. Uniform NiCo-LDH nanosheets were synthesized on a nickel foam substrate using a hydrothermal method and used as the anode electrocatalyst.

[0041] Electrochemical performance tests were conducted in a flow-through electrolyzer using 1M KOH containing 0.5M methanol as the electrolyte and a Shanghai Chenhua CHI 760E workstation. In short, a gas diffusion electrode coated with a Bi-based catalyst was used as the working electrode, Ag / AgCl as the reference electrode, and nickel foam loaded with NiCo-LDH nanosheets as the counter electrode. Polarization curves were measured. For the full electrolyzer, the reference electrode was clamped onto the counter electrode, and the presence or absence of methanol in the electrolyte was compared by measuring the polarization curves. The results showed that the addition of methanol significantly reduced the full electrolyzer voltage compared to the conventional OER reaction, thanks to the lower onset and operating potentials of methanol oxidation. Further comparisons of polarization curves with and without an ion-exchange membrane revealed that the absence of a membrane effectively eliminated interfacial resistance and mass transfer resistance, further reducing the power input; a voltage of only 2.52V was required to achieve a 400mA cm⁻¹. -2 The industrial current density is achieved, saving approximately 350mV of voltage compared to a conventional CO2RR / / OER configuration. Finally, the stability of the device was analyzed through constant current testing. 1 Analysis of the products by H NMR spectroscopy revealed that the liquid products produced by both the cathode and anode were formic acid, with a Faraday efficiency exceeding 93%.

[0042] Example 3

[0043] SnS / aminocarbon was prepared using an improved hard template-induced method. First, ammonium molybdate (1.96 g) was dissolved in deionized water containing nitric acid via a hydrothermal method. The solution was then kept at 200 °C for 20 hours in a 50 mL reactor, followed by centrifugation, washing, and drying. Next, polyvinylpyrrolidone (1.5 g), glycerol (0.08 g), acetamide (0.035 g), and MoO3 (0.04 g) were dissolved in ethanol and heated to 60 °C for 1 hour. Simultaneously, SnCl2·2H2O (2.9 g) and thioacetamide (0.96 g) were dissolved in ethylene glycol (32 mL) to ensure a Sn to S molar ratio of 1:1. Then, the prepared ethylene glycol solution containing SnCl2 (3.2 mL) and the ethylene glycol solution containing thioacetamide (3.2 mL) were added to the prepared mixture containing PVP, glucose, acetamide, and MoO3, respectively, and the mixture was stirred for 5 minutes to ensure thorough mixing. Subsequently, the resulting mixture was sealed in a Teflon-lined stainless steel autoclave and heated to 160°C for 24 hours. Next, the suspension was centrifuged, and the precipitate was washed three times with ethanol and dried at 65°C for 12 hours to obtain MoO3 / SnS / amino-modified-C. The obtained MoO3 / SnS / amino-modified-C (0.10 g) was then dissolved in ethanol (50 mL), and ammonia (28%, 2 mL) was added. The MoO3 template was etched for 2 hours with stirring. Finally, the resulting suspension was centrifuged, and the precipitate was calcined together with ethanol at 500°C for 1 hour in an argon atmosphere to obtain the final material. This catalyst achieves a Faraday efficiency of up to 92.6% for formate and a formate bias current density of up to 41.1 mA cm⁻¹. -2 This catalyst was drop-coated onto a gas diffusion layer using Nafion and used as a cathode catalyst; NiS nanowires were prepared by direct gas-phase sulfidation of nickel foam and used as an anode for the electrocatalysis of ethylene glycol oxidation.

[0044] Electrochemical performance testing and process feasibility analysis were conducted in a flow-through electrolyzer using 1M KOH containing 0.5M ethylene glycol as the electrolyte. Specifically, a gas diffusion electrode coated with SnS / aminocarbon catalyst was used as the working electrode, and nickel foam loaded with NiS nanowires was used as the counter and reference electrodes. Polarization curves and stability tests were performed. The results showed that the addition of ethylene glycol also achieved energy savings, specifically a significant reduction in the full-cell voltage required for the same current density, exceeding 300 mV compared to the OER reaction. This indicates that the electro-oxidation of ethylene glycol is thermodynamically more favorable than OER and has a faster reaction kinetic rate. Separate tests at the cathode and anode confirmed that the addition of ethylene glycol and the presence of CO2 did not reduce the catalytic activity and selectivity of the anode and cathode catalysts. Full-cell tests compared the polarization curves under conditions with and without ion exchange membranes. The membrane-free device achieved 100 and 400 mA cm⁻¹ with voltages of only 2.16 V and 2.65 V, respectively. -2 The current density was reduced, saving 200–300 mV of battery voltage compared to the conventional anodic OER reaction configuration, and reducing the voltage by 100–200 mV compared to the configuration with a separator. To determine the anode and cathode products, potentiostatic tests were performed on the anode and cathode catalysts, and qualitative and quantitative analyses were conducted using gas chromatography and nuclear magnetic resonance spectroscopy. The Faradaic efficiency of formic acid in both the anode and cathode exceeded 90%. Product testing after full-cell potentiostatic testing revealed that the only liquid product was formic acid (…). Figure 5 In the process, the hydroxyl group of ethylene glycol underwent oxidation to the carboxyl group and the C-C bond was broken. Formic acid, as the final product, further confirmed the feasibility and economy of the process.

[0045] Example 4

[0046] First, using bismuth nitrate (1.940 g) and stannous chloride (1.139 g) as precursors, deionized water as solvent, and sodium hydroxide to adjust the pH to 12, the mixture was stirred for 10 minutes and reacted at 180 °C for 24 hours via a solvothermal method. After cooling, centrifugation, washing, and drying, BiSn bimetallic oxide was synthesized. Then, it was electrochemically reduced in 0.5 M KHCO3 electrolyte at an operating potential of -0.8 V vs. RHE for 1–2 hours to prepare Bi@Sn nanoparticles with a core-shell structure. Basic electrochemical tests were performed by drop-coating these nanoparticles onto carbon paper. At an operating potential of -1.1 V vs. RHE, the formate Faradaic efficiency reached 91%, and the formic acid deflection current density reached 45 mA cm⁻¹. -2 Further performance tests were conducted in a flow cell, and the catalytic performance of Bi@Sn NPs was evaluated using chronopotential measurement. The catalytic performance was assessed within the range of -25.0 to 200 mA / cm². -2 Within the range, the FE of Bi@Sn NPs HCOOHThe yield exceeded 92%, with corresponding cathode potentials ranging from -0.81 to -1.15 V. For the anode catalyst, a NiMoO4 catalyst with a nanowire structure was prepared by hydrothermal synthesis using nickel nitrate and sodium molybdate as precursors, maintained at 150 °C for 6 hours, followed by washing and drying.

[0047] The prepared powdered Bi@Sn NPs catalyst was drop-coated onto a gas diffusion layer as the cathode catalyst, and the nickel foam-supported NiMoO4 catalyst was used as the anode catalyst. A flow-through electrolyzer was then assembled using 0.3M glycerol and 1M KOH as the electrolyte. Electrochemical performance was tested using a Shanghai Chenhua CHI 760E workstation. The gas diffusion electrode served as the working electrode, and the catalyst-supported nickel foam served as the counter and reference electrodes. CV and LSV tests were performed. Firstly, the addition of glycerol significantly reduced the full-cell voltage required to achieve the same current density compared to before the addition, demonstrating a significant energy-saving effect. Furthermore, the catalytic kinetics of glycerol oxidation were faster than those of OER. In addition, the polarization curves under ion-exchange membrane and non-ion-exchange membrane conditions were compared. The membrane-free device was relatively more stable, avoiding the swelling of the ion-exchange membrane caused by the presence of glycerol, and eliminating interfacial resistance and mass transfer resistance, resulting in higher energy efficiency and improved operating cycle. A 200mA cm⁻¹ current was achieved with only 2.32V. -2 The industrial current density is reduced by approximately 200mV compared to a conventional CO2RR / / OER configuration. 1 The analysis of the products by 1H NMR spectroscopy further confirmed the feasibility of the invention. The electrolyte contained only formic acid, with no other products. Therefore, the separation process is simpler and less costly than other anodic oxidation reactions. In summary, compared with traditional CO2 reduction and anodic OER configuration, the proposed method of replacing the OER reaction with small molecule alcohol oxidation significantly reduces process energy consumption and yields formate products with a much higher added value than oxygen, providing a completely new approach for the industrial application of carbon dioxide electroreduction.

[0048] Example 5

[0049] Due to the excellent salting-out properties of Bi-Sn hydrogels, sodium citrate (0.5 M), bismuth nitrate (0.025 mM), tin chloride (0.025 mM), and NaBH4 aqueous solution (1 mM) were added sequentially to 15 mL of water under stirring conditions, using NH4Cl as the initiator and NaBH4 as the reducing agent. Then, NH4Cl (2.49 M) was added to the prepared 15 mL solution, followed by standing with 6% nitric acid to form a monolithic Bi-Sn hydrogel. The resulting hydrogel was then washed repeatedly with deionized water and freeze-dried using a freeze dryer to prepare a bismuth and tin bimetallic non-noble metal-based aerogel with a three-dimensional morphology and abundant interfaces. Electrochemical tests were then conducted in 0.5 M KHCO3 electrolyte and an H-type electrolytic cell. The results showed that the designed Bi-Sn aerogel exhibited excellent electrocatalytic performance and selectivity for the reduction of CO2 to formic acid, with a Faradaic efficiency of up to 93.9%, comparable to most existing catalysts. The NiMoO4 material from Example 4 was thermally annealed under an ammonia atmosphere (500°C for 2 hours) to obtain Ni-Mo-N nanosheets supported on a nickel foam substrate. Electrochemical tests were conducted in a potassium hydroxide (1M KOH) electrolyte containing 0.5M methanol, demonstrating excellent catalytic performance; a 400 mA cm⁻¹ catalytic flux was achieved at only 1.45 V. -2 The current density was high, and the selectivity for formic acid reached 95%.

[0050] The prepared Bi-Sn aerogel catalyst was drop-coated onto a gas diffusion layer and used as the cathode catalyst (working electrode). A Ni-Mo-N catalyst supported on nickel foam was used as the anode catalyst. Electrochemical performance and device feasibility were evaluated in a flow-through electrolyzer using 0.5M methanol and 1M KOH as the electrolyte. The results showed that the addition of methanol significantly reduced electrode polarization and cell voltage. Compared to the OER and CO2RR configurations, the full-cell voltage required to achieve the same current density was significantly reduced, resulting in an energy saving of approximately 300mV. Further investigation into the feasibility and stability of the device without an ion-exchange membrane revealed that both the cathode and anode products were formic acid, and the presence of methanol did not harm the performance of the cathode catalyst. The cathode and anode could share the same electrolyte; therefore, removing the ion-exchange membrane did not affect catalytic performance or cause product cross-contamination, nor did it increase the complexity of the separation process or cost. This allowed for double the production of potassium formate. Furthermore, the membrane-free device was relatively more stable, avoiding swelling of the ion-exchange membrane caused by methanol, eliminating the need for periodic replacement, saving operating costs, and extending the device's operating cycle. Even more interestingly, by eliminating interfacial resistance and mass transfer resistance, a 400mA cm-current impedance is achieved with only 2.63V. -2The industrial current density is higher than that of conventional CO2RR / / OER configurations, making it more energy-efficient. Overall, this process offers a more significant advantage in energy efficiency, achieving double the production of potassium formate without increasing separation costs or complexity, further enhancing added value. Most importantly, it avoids the use of ion exchange membranes, saving operating costs, extending the unit's operating cycle, and providing a new approach to carbon utilization.

Claims

1. A process for the membrane-free co-production of formate via carbon dioxide electroreduction coupled with alcohol oxidation, characterized in that: A flow-through electrolytic cell was used as the electrolysis equipment, and the co-production of formate was achieved by coupling cathode carbon dioxide reduction with anodic alcohol oxidation. The flow-through electrolyzer includes a gas chamber, a cathode chamber, and an anode chamber. A gas diffusion electrode is used to separate the gas chamber and the cathode chamber. No ion exchange membrane is installed in the flow-through electrolyzer. The cathode electrolyte and the anode electrolyte are the same alcohol-containing potassium hydroxide or sodium hydroxide electrolyte. The alcohol is methanol, ethylene glycol, or glycerol. The catalyst for cathodic reduction is selected from metal nanoarrays, alloys, metal oxides, sulfides, organometallic frameworks, and carbon-coated metal nanomaterials containing bismuth, tin, antimony, and indium; the Faraday efficiency of the catalyst for cathodic reduction to formic acid should be greater than 80%. The anode catalyst uses oxides and hybrids, alloys, nanoarrays, and organometallic frameworks containing metallic iron, cobalt, nickel, copper, and molybdenum. The anode catalyst has a Faradaic efficiency of more than 80% for formate, and formate is the only product of the oxidation of small molecule alcohols. The electrolysis process is as follows: the captured CO2 is introduced into the cathode gas chamber, and the electrolyte containing small molecule alcohols enters the anode chamber and cathode chamber respectively; on the cathode side, CO2 gains electrons at the gas-liquid-solid three-phase interface through the gas diffusion electrode and undergoes a reduction reaction to obtain formic acid, while on the anode side, the small molecule alcohols lose electrons and are oxidized to formic acid. The entire electrolysis process avoids the use of ion exchange membranes, the voltage range is 2~5 V, and the electrolysis process is carried out continuously. Product separation section: For gaseous products, pressure swing adsorption is used to recover and recycle unreacted carbon dioxide, while liquid products need to be neutralized with formic acid and distilled under reduced pressure to obtain high-purity formate products. The cathode catalyst is selected from Bi nanosheets, BiSn alloy nanospheres, SnS / aminocarbon, In-MOF, Bi-MOF, and BiSn aerogel; the anode catalyst is a nickel-based nanomaterial selected from nickel hydroxide, nickel sulfide, nickel nitride hybrids, nickel-molybdenum, nickel-cobalt alloy oxides, NiCo-MOF, NiCo-LDH, and Ni-Mo-N nanosheets.

2. The process for co-producing formate by carbon dioxide electroreduction coupled with alcohol oxidation without a diaphragm, as described in claim 1, is characterized in that: For the gaseous products generated by cathodic electroreduction, when the formic acid Faraday efficiency is greater than 90%, carbon dioxide is recovered by pressure swing adsorption, while hydrogen and carbon monoxide products are collected as syngas without separation.

3. The process for co-producing formate by carbon dioxide electroreduction coupled with alcohol oxidation without a diaphragm, as described in claim 1, is characterized in that: It uses renewable and clean energy sources such as solar and wind power to directly supply power to the electrolysis without the need for voltage boosting.

4. The process for producing formate by carbon dioxide electroreduction coupled with alcohol oxidation without a diaphragm, as described in claim 1, is characterized in that: The product of CO2 reduction at the cathode is formic acid, and the alcohol oxidized at the anolyte is methanol, ethylene glycol, or glycerol. The final product of its oxidation is also formic acid. The selectivity of the cathode and anode products should exceed 80%.

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