Electrode material for the electrochemical reduction of carbon dioxide, electrode and electrochemical cell comprising such an electrode material

Abstract

The invention relates to an electrode material based on cobalt phosphides for the electrochemical reduction of carbon dioxide and a process for the electrochemical reduction of carbon dioxide to (C 2+ )-products using the electrode material. The plan is to use the regeneratively obtained monomers for the production of polymers as materials for industrial applications, especially in the automotive industry.

Description

[0001] The invention relates to an electrode material based on cobalt phosphides for the electrochemical reduction of carbon dioxide, an electrode comprising such an electrode material, and an electrochemical cell with such an electrode. The invention further relates to a process for the electrochemical reduction of carbon dioxide to (C 2+ ) products using the electrode material. The invention further relates to the use of the (C 2+ ) products for the production of thermoplastic polymers.

[0002] Plastics are organic polymers that can be processed in a variety of ways. Their technical properties, such as formability, hardness, elasticity, strength, heat resistance, and chemical resistance, can be varied within wide limits through the choice of raw materials, manufacturing processes, and the addition of additives. Plastics are lighter and more economical than many other materials. These properties, along with their extremely flexible and easy processability, make them the first choice for many industrial and commercial applications. Since the widespread availability of petroleum in the early 20th century, most traditional plastics have been made from this fossil raw material.

[0003] Sustainable polymer solutions have recently gained increasing importance in the automotive industry. The three most relevant raw material sources for sustainable polymers include bio-based approaches based on renewable resources, recycled plastics obtained from recycling processes, and CO2-based polymer approaches. A variety of bio-based polymers are already known from the state of the art, such as polylactide (PLA) and polyhydroxybutyrate (PHB). Furthermore, thermoplastic polymers from recycled processes are increasingly being used to minimize the CO2 impact through a closed material cycle.However, these two polymer classes are not suitable for use in applications with demanding requirements, since neither the bio-based approaches nor the recycled plastics allow for control of the molecular weight and thus the physical, mechanical and chemical properties, either through natural synthesis processes or through degradation effects in the recycling process.

[0004] The electrochemical reduction of carbon dioxide (CO2) powered by renewable electricity represents a promising alternative for the production of CO2-neutral fuels and chemical feedstocks in a sustainable cycle. CO2 reduction using water as a hydrogen source can be carried out catalytically in electrolysis cells that utilize renewable electricity. However, the commercial development of this technology is hampered by low energy efficiency and poor product selectivity.

[0005] One approach is disclosed, for example, in WO 2019 / 048878 A1, which proposes the polymerization of CO2 with an epoxide in the presence of an organometallic catalyst, using nickel or cobalt as examples. This polymerization can be used to produce polycarbonates, polyethercarbonate polyols, or polyester polyols, which serve as starting materials for polyurethanes. However, a higher CO2 content can lead to a deterioration in the material properties.

[0006] Recent research in the field of CO2 reduction has shown that, for example, hydrocarbons, carboxylic acids, aldehydes, ketones, ethers, alcohols, and mixtures thereof can be produced using NiP and / or FeP-based catalysts. Overall, this new technology represents a promising approach for minimizing CO2 emissions into the atmosphere and slowing global warming. For example, WO 2021 / 236746 A1 discloses the production of formic acid, ethylene glycol, methylglyoxal, and 2,3-furandiol from carbon dioxide using NiP as a catalyst. The NiP catalyst is used together with a co-catalyst, such as a Lewis or Brønsted acid, Lewis base, Brønsted base, or mixtures thereof. Alternatively, a mixture of NiP / FeP is described as a catalyst. S. Banerjee et al. (ACS Catal.2021, 11, 11706-11715) describes, for example, the mechanistic processes of the electrochemical CO2 reduction of Ni2P and the selectivity with regard to the formation of formaldehyde.

[0007] M. Usman et al. (Nanomaterials 2021, 11, 2029) also describe the electrochemical conversion of CO2 to fuels using cobalt-based catalysts. The co-catalysts investigated include metallic cobalt, Co-metal mixtures, Co-complexes, Co-based organometallic compounds, Co-based covalent organic compounds, Co-nitrides, and Co-oxides.

[0008] Another approach is the photocatalytic conversion of CO2 to CO, reported by P. Niu et al. (ChemSusChem 2021, 14, 1 - 7). Nanoparticulate cobalt phosphide (CoP-600) is used as a co-catalyst, which is obtained by reacting Co3O4 as a precursor and NaH2PO2 · H2O as a phosphorus source.

[0009] However, the use of various catalysts such as NiP and FeP in electrochemical and photochemical processes usually requires high overpotentials, which makes the process inefficient and uneconomical.

[0010] Overall, the solutions known from the state of the art lead to additional byproducts that cannot be used in the polymer chemistry industry. Other solutions in the field of electrochemical catalysis reduce CO2 to CO, which cannot be polymerized directly. This requires additional energy-intensive intermediate steps, which would result in a higher carbon footprint compared to direct synthesis.

[0011] The synthesis of transition metal phosphides, such as nickel or cobalt phosphides, can be achieved using a hydrothermal or solvothermal process. For example, US 2018 / 0087163 A1 describes the production of an electrode material for the production of hydrogen from water. Elemental phosphorus is converted into the corresponding transition metal phosphides with a transition metal at temperatures of 300 to 1100°C and in an inert atmosphere. In addition to nickel and copper phosphides, cobalt phosphides such as CO are also used. 1,94 P, Co 1,85 P, Co2P, CoP, CoP2, CoP3, CoP4, and mixtures thereof. CN 108878164 A, on the other hand, describes the preparation of a nickel phosphide starting from red phosphorus and a nickel salt, such as nickel nitrate, nickel acetate, or nickel chloride. The resulting nickel phosphide is used as an electrode material for lithium batteries.

[0012] The invention is based on the object of providing a novel electrode material for selective CO2 reduction for the production of starting products for polymer synthesis, whereby the polymers thus obtained can be used as thermoplastic materials for the production of materials that can be used, for example, in the automotive industry.

[0013] The above object is achieved by an electrode material comprising at least one cobalt phosphide catalyst of the general empirical formula Co x P y , where x represents a number from 1 to 2 and y represents a number from 1 to 5. In particular, the number is always an integer.

[0014] The object is further achieved by an electrode comprising the electrode material according to the invention. The electrode is preferably a cathode.

[0015] The object is further achieved by an electrochemical cell comprising the electrode according to the invention. The electrochemical cell is, in particular, an electrolyzer, and preferably a zero-gap electrolyzer.

[0016] Furthermore, the object is achieved by a process for the electrochemical reduction of carbon dioxide, comprising a cathode comprising the electrode material according to the invention. The process comprises supplying gaseous carbon dioxide or carbon dioxide in aqueous solution and reducing the carbon dioxide at the cathode to at least one (C 2+ )-Product.

[0017] By using cobalt phosphide (Co x P y) compounds as catalysts in the electrochemical reduction of carbon dioxide, a sufficiently high coulombic efficiency (> 60%) can be achieved, which greatly reduces the energy demand for electrolysis. The (C 2+) products are monomers that can be further processed into thermoplastic polymers. This allows thermoplastic polymers with a defined molecular weight and a defined molecular weight distribution to be produced, and their property profile can be tailored to the respective application. The polymers produced in this way do not differ chemically from petrochemical plastics. The use of monomers obtained by electrochemical reduction of carbon dioxide as starting materials in the production of thermoplastic polymers thus offers the advantage of a negative CO2 balance compared to petrochemical polymer solutions. In particular, the use of carbon dioxide from the atmosphere for electrochemical reduction not only reduces carbon dioxide emissions but also compensates for unavoidable carbon dioxide emissions.One way to offset these carbon dioxide emissions is to capture carbon dioxide from the ambient air. This process is also known as direct air capture.

[0018] In a preferred embodiment of the invention, the cobalt phosphide (Co x P y ) catalyst is selected from the group consisting of Co2P, CoP, CoP2, Co2P3, Co2P3, CoP3, CoP4, or a mixture of two or more of these. The use of the cobalt phosphide catalyst supports the optimal binding of carbon dioxide for electrochemical reduction.

[0019] In an alternative embodiment of the invention, it is provided that the electrode material comprises one or more metals and / or their salts or oxides selected from copper (Cu), silver (Ag), gold (Au), zinc (Zn), nickel (Ni), platinum (Pt) as co-catalyst. The use of (Co x P y) as a catalyst in combination with a co-catalyst enables high efficiency and product selectivity in the production of (C 2+) products, such as monoethylene glycol. The co-catalyst binds to a reaction intermediate on the surface, thereby influencing the bond orientation of the intermediate. Furthermore, the intermediate is activated for subsequent reaction with the surface-bound hydrides. The bond strength of the intermediate is also influenced, facilitating the formation of new reaction intermediates on the surface. The presence of such a co-catalyst thus changes the selectivity of the reaction by altering the populations and binding affinities of the reaction intermediates on the surface of the catalyst. The use of co-catalysts can therefore increase the selectivity and Coulombic efficiency and reduce the energy consumption during electrolysis.

[0020] In a preferred embodiment of the invention, the electrode material comprises a conductive filler. The use of a conductive filler can increase the current density.

[0021] Carbon dioxide for the process according to the invention is preferably extracted from atmospheric air, in particular by means of a direct air capture process. This can improve the CO2 balance. Furthermore, direct air capture processes, in which carbon dioxide is reversibly bound to solid adsorbers, can be carried out simply and cleanly using exclusively physical means.

[0022] In a preferred embodiment of the invention, a gas diffusion electrode is used for the electrochemical reduction of carbon dioxide. Due to the comparatively slow diffusion processes and the low solubility of carbon dioxide in water, it is advantageous to use a gas diffusion electrode.

[0023] In a further advantageous embodiment of the invention, the aqueous solution comprises KHCO3 as the catholyte. By using KHCO3, the solution resistance can be reduced due to the increased conductivity, thus minimizing resistance losses, which leads to a stable solution resistance during electrolysis. Furthermore, the KHCO3 leads to an alkaline pH, which has a positive effect on selectivity.

[0024] In a further preferred embodiment of the invention, the electrochemical reduction takes place in a zero-gap electrolyzer. A major problem with electrochemical CO2 reduction is the transport of carbon dioxide to the electrode surface, since the reaction can only take place in close proximity (a few nanometers). In the zero-gap electrolyzer, the carbon dioxide does not need to be dissolved beforehand, but is delivered to the electrode in pure form, thus minimizing the influence of diffusion on the reaction rate.

[0025] Further preferred embodiments of the invention emerge from the remaining features mentioned in the subclaims.

[0026] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0027] The individual steps of the method according to the invention are described in more detail below: A) Production of carbon dioxide: The carbon dioxide required for the production of thermoplastic polymers can, for example, be captured from the atmosphere. In principle, systems and processes for capturing carbon dioxide from the ambient air are known. Such capture can be carried out, for example, using the so-called "direct air capture" process, in which the carbon dioxide is captured directly from the ambient air and fed into a subsequent process. For example, the atmospheric air can be passed through an adsorber that reversibly binds CO2 while allowing the remaining air constituents to pass through. By increasing the temperature, the bound CO2 can be released again and fed into the subsequent process.It is also possible to obtain the required carbon dioxide from point sources, such as the post-combustion process, in which flue gases are washed with a liquid solvent, the oxyfuel process or the pre-combustion process, in which the carbon dioxide is separated before the combustion process. B) Production of the (Co x P y ) catalyst: The electrochemical reduction of carbon dioxide to (C 2+ ) products is carried out by using a cobalt phosphide (Co x P y ) Compound as a metal phosphide catalyst, where x can be a number between 1 and 2 and y can be a number between 1 and 5. In one embodiment of the invention, the CoP is selected from the group Co2P, CoP, CoP2, Co2P3, Co2P3, CoP3, CoP4.

[0028] The production of the (Co x P y) compound is prepared starting from either elemental cobalt or CoCl2 and elemental phosphorus at 100°C to 700°C for 18 to 24 h in vacuum-sealed quartz tubes. C) Electrochemical reduction of carbon dioxide: The electrochemical reduction of carbon dioxide can, for example, take place at solid-state electrodes in aqueous electrolyte solutions. The electrolysis cell for the electrochemical reduction of CO2 can comprise a cathode, anode and an electrolyte, with the cathode containing a (Co x P y ) catalyst.

[0029] The cathode preferably comprises, in addition to the (Co x P y) catalyst additionally contains a co-catalyst. Suitable co-catalysts are, for example, salts or oxides selected from Cu, Ag, Au, Ni, Pt, Zn or mixtures thereof. The salts or oxides become soluble at a suitable pH value. Alternatively, metals selected from Cu, Ag, Au, Ni, Pt, Zn or their alloys can be used, for example as nanoparticles. The co-catalysts can be applied to the surface of the catalyst or incorporated into the surface or mass of the catalyst by doping. The salts of the co-catalysts can also be dissolved in the electrolyte solution.

[0030] The cathode is the electrode where the reductive half-reaction takes place. It can be a gas diffusion electrode, a porous electrode, a solid electrode, or a bulk electrode.

[0031] The corresponding cathodes can also contain materials commonly used in cathodes, such as binders, ionomers, for example, anion-conductive ionomers, fillers, or hydrophilic additives. Suitable binders include, for example, organic binders selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, NAFION is preferred ® used as a binder. Conductive fillers such as carbon can also be included in the cathode.

[0032] In a first variant (variant I), the cathode is obtained by heating the Co x P y- Catalyst is mixed with the support material, such as a Nafion solution (5-10 wt.%, preferably 5 wt.%) and pressed to form the electrode. Additionally, one or more co-catalysts can be used in combination. The pressing process can be carried out using a selected binder, such as NAFION. ® , PTFE, PFVD (1 - 5 wt%, preferably 1 wt%).

[0033] In a second variant (variant Ila), a suspension comprising the Co x P y- catalyst and at least one solvent, with which a platelet made of glassy carbon, nickel, or porous metallic materials is coated. Suitable solvents include alcohols such as methanol, ethanol, or isopropanol, or mixtures thereof with water. The coating can be carried out via drop, dip, spin, or spray coating. In addition, one or more co-catalysts can be used in combination. The coating process can be carried out using a selected binder, such as NAFION. ® , PTFE, PFVD (1-5 wt%). Conductive carbon black (10-90 wt%) can also be added.

[0034] It is also possible to carry out the process according to the invention (variant IIb) using a gas diffusion electrode (GDE). Gas diffusion electrodes are electrodes in which liquid, solid, and gaseous phases are present, and where, in particular, a conductive catalyst catalyzes an electrochemical reaction between the liquid and gaseous phases. In variant IIb, a gas diffusion electrode is formed by coating a gas diffusion layer (GDL), such as a carbon cloth or carbon paper; metal foam, such as titanium foam, to which a microporous layer can additionally be applied, with a Co x P y-containing suspension. Suitable solvents include alcohols such as methanol, ethanol, or isopropanol, or mixtures thereof with water. Additionally, one or more co-catalysts can be used in combination. The coating process can be carried out using a selected binder, such as Nafion, PTFE, or PFVD (1–5 wt%). A conductive carbon black (10–90 wt%) can also be added. The suspension is then applied to the GDL. The coating can be applied using drop, dip, spin, or spray coating, as well as screen printing.

[0035] A preferred embodiment of variant IIb is a zero-gap electrolyzer. A gas diffusion electrode is pressed against an anion or cation exchange membrane as the cathode. On the opposite side of the membrane, an anode is pressed against the membrane. The anode can be, for example, IrO. x-coated carbon cloth or paper, as well as other conductive materials. The cathode chamber is then flushed with gaseous CO2, which can be humidified and tempered beforehand. A liquid anolyte is passed through the anode chamber. The anolyte can be an aqueous, organic, or ionic liquid, to which conductivity-enhancing additives such as KOH can also be added.

[0036] The electrochemical cell further contains an electrolyte solution comprising an anolyte and a catholyte. The anolyte can be an aqueous solution of an alkali metal salt, such as KOH. An aqueous potassium bicarbonate solution, preferably 0.5 M KHCO3, can be used as the catholyte, for example.

[0037] The oxidation of a substance takes place in the anode chamber at the anode, which is electrically connected to the cathode by a power source that provides the voltage for electrolysis. Furthermore, the anode material is not particularly limited and depends primarily on the desired reaction. Examples of anode materials include platinum or platinum alloys, palladium or palladium alloys, and glassy carbon. Other anode materials include conductive oxides such as doped or undoped TiO2, indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), iridium oxide, etc. These catalytically active compounds can also be applied to the surface using thin-film technology, for example, on a titanium and / or carbon support. Platinum (Pt) is preferably used as the anode, with an aqueous solution of KOH as the anolyte.

[0038] By applying a voltage to the anode and cathode, CO2 is reduced to a product mixture by means of the process according to the invention, which (C 2+ ) products, such as ethane, ethene, ethyne, ethanol, monoethylene glycol, acetaldehyde, acetic acid, furandiol, or oxalic acid, preferably diol structures, which can serve as monomers. Monomers can be, for example, saturated and unsaturated hydrocarbons, alcohols, carboxylic acids, and acid anhydrides, which can be further processed into thermoplastic polymers.

[0039] In a preferred embodiment, the (C 2+ ) products are used to produce thermoplastic polymers for industrial applications, with particular preference for use in the automotive industry.

[0040] The process according to the invention can thus be used to produce starting materials for materials such as textiles, covers, belts, airbags, bearings, gears, cams, plugs, screws, nuts and other plastic components not explicitly mentioned.

[0041] Other areas of application include continuous filaments, staple fibers, crimped fibers, felts or tows for clothing fabrics, home textiles and industrial purposes or in blends with other fibers such as cotton, wool, silk and synthetic fibers, which can be processed into textiles in the form of woven and knitted fabrics.

[0042] Other areas of application include fine chemicals, floor coverings, and technical applications such as conveyor belts, drive belts, tire cord, fire hoses, ropes, filter materials, sewing threads and yarns, as well as gas-tight films and gas-tight bottles for the food industry. Further applications include equipment construction, precision engineering, household appliances, photographic film, magnetic tape, and food packaging.

[0043] The invention is explained below in exemplary embodiments. Production of the catalyst Co x P y :Synthesis pathway I:

[0044] Cobalt is mixed with a 1.5 mol% excess of red phosphorus in an agate mortar. The mixture is then transferred to a quartz glass ampoule. After repeated evacuation and subsequent purging with argon, the ampoule is thermally sealed under vacuum. The sealed ampoule containing the reaction mixture is then heated in an oven at 700 °C for 24 hours. After cooling, the resulting Co x P y removed from the ampoule and characterized. Synthesis route II:

[0045] Cobalt chloride (CoCl2) is mixed with a stoichiometric amount of red phosphorus in an agate mortar to obtain the desired Co x P y and PCl3. The following example reaction equation for CoP2 illustrates the reaction pathway: 3 CoCl2 + 8 P → 3 CoP2 + 2 PCl3

[0046] The mixture is then transferred to a borosilicate glass ampoule and thermally sealed under vacuum. The ampoule is then heated to 500 °C at a heating rate of 100 °C / h. This temperature is then maintained for 18–24 h. After the reaction time has elapsed, the ampoule is withdrawn halfway from the furnace, causing the formed PCl3 to condense on the cooling glass wall. Once no further condensation is observed, the ampoule can be completely cooled to room temperature and opened. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2019 / 048878 A1

[0005] WO 2021 / 236746 A1

[0006] US 2018 / 0087163 A1

[0011] CN 108878164 A

[0011] Cited non-patent literature

[0000] S. Banerjee et al. (ACS Catal. 2021, 11, 11706-11715

[0006] M. Usman et al. (Nanomaterials 2021, 11, 2029

[0007] P. Niu et al. (ChemSusChem 2021, 14, 1 - 7

[0008]

Claims

[1] Electrode material for the electrochemical reduction of carbon dioxide, comprising at least one cobalt phosphide catalyst of the general formula Co x P y , where x stands for a number from 1 to 2 and y stands for a number from 1 to 5. [2] Electrode material for the electrochemical reduction of carbon dioxide according to claim 1, characterized by that the cobalt phosphide catalyst is selected from the group Co2P, CoP, CoP2, Co2P3, Co2P3, CoP3, CoP4. [3] Electrode material for the electrochemical reduction of carbon dioxide according to claim 1 or 2, characterized by that the electrode material comprises at least one metal and / or salts or oxides thereof as co-catalyst, selected from copper (Cu), silver (Ag), gold (Au), zinc (Zn), nickel (Ni), platinum (Pt). [4] Electrode material for the electrochemical reduction of carbon dioxide according to one of claims 1 to 3, characterized bythat the electrode material comprises a conductive filler. [5] Electrode, in particular cathode, comprising an electrode material according to one of claims 1 to 4. [6] Electrochemical cell, in particular electrolyzer, preferably zero-gap electrolyzer, comprising an electrode according to claim 5. [7] A process for the electrochemical reduction of carbon dioxide using an electrode according to claim 5 or an electrochemical cell according to claim 6, supplying gaseous carbon dioxide or carbon dioxide in aqueous solution and reducing the carbon dioxide at the cathode to a (C 2+ )-Product. [8] Method according to claim 7, characterized by that the carbon dioxide is extracted from atmospheric air. [9] Method according to claim 8, characterized bythat the extraction of carbon dioxide is carried out by means of a direct air capture process from the atmospheric air or from point sources. [10] Method according to one of claims 7 to 9, characterized by that the aqueous solution comprises KHCO3 as catholyte. [11] Method according to one of the preceding claims 7 to 10, characterized by that the cathode is designed as a gas diffusion electrode. [12] Method according to one of claims 7 to 11, characterized by that the (C 2+ ) product comprises ethane, ethene, ethyne, ethanol, monoethylene glycol, acetaldehyde, acetic acid, furandiol and / or oxalic acid. [13] (C 2+ ) product produced by a process according to any one of claims 7 to 12. [14] Use of a (C 2+ ) product according to claim 13 for the production of thermoplastic polymers for industrial applications, in particular in the automotive industry.