GAS DIFFUSION ELECTRODE FOR REDUCING CARBON DIOXIDE

Abstract

The invention relates to a gas diffusion electrode for reducing carbon dioxide, having a special catalyst morphology (silver in the form of agglomerated nanoparticles having a BET surface area of at least 2 m2/g), and to an electrolysis device. The gas diffusion electrode comprises at least one carrier and a porous coating on the basis of an electrochemically active porous silver catalyst and a hydrophobic material. The invention further relates to a production method for the gas diffusion electrode and to the use thereof as a carbon dioxide GDE in e.g. chlorine electrolysis.

Claims

1.-14. (canceled)

15. A gas diffusion electrode for reducing carbon dioxide, comprising at least one sheet-like, electrically conductive support and a gas diffusion layer and electrocatalyst applied to the support, wherein the gas diffusion layer consists at least of a mixture of electrocatalyst and a hydrophobic polymer and silver acts as electrocatalyst, and wherein the electrocatalyst contains silver in the form of highly porous agglomerated nanoparticles and said nanoparticles have a surface area measured by the BET method of at least 2 m.sup.2/g.

16. The gas diffusion electrode as claimed in claim 15, wherein the proportion of electrocatalyst is from 80 to 97% by weight, based on the total weight of electrocatalyst and hydrophobic polymer.

17. The gas diffusion electrode as claimed in claim 15, wherein the proportion of hydrophobic polymer is from 20 to 3% by weight, based on the total weight of electrocatalyst and hydrophobic polymer.

18. The gas diffusion electrode as claimed in claim 15, wherein the silver particles are present as agglomerate having an average agglomerate diameter in the range from 1 to 100 m.

19. The gas diffusion electrode as claimed in claim 15, wherein the silver nanoparticles have an average diameter in the range from 50 to 150 nm.

20. The gas diffusion electrode as claimed in claim 15, wherein electrocatalyst and hydrophobic polymer have been applied in powder form to the support and compacted and form the gas diffusion layer.

21. The gas diffusion electrode as claimed in claim 15, wherein the hydrophobic polymer is a fluorine-substituted polymer.

22. The gas diffusion electrode as claimed in claim 15, wherein the electrode has a total loading of catalytically active component in the range from 5 mg/cm.sup.2 to 300 mg/cm.sup.2.

23. The gas diffusion electrode as claimed in claim 15, wherein the support is based on nickel, silver or a combination of nickel and silver.

24. The gas diffusion electrode as claimed in claim 15, wherein the support is present in the form of a gauze, woven mesh, formed-loop knit, drawn-loop knit, nonwoven, expanded metal or foam.

25. The use of the gas diffusion electrode as claimed in claim 15 for the electrolysis of carbon dioxide to give carbon monoxide.

26. A process for the electrochemical conversion of carbon dioxide into carbon monoxide, wherein the carbon dioxide is reacted cathodically at a gas diffusion electrode as claimed in claim 15 to form carbon monoxide, and chlorine or oxygen is simultaneously produced on the anode side.

27. The process as claimed in claim 26, wherein the current density in the reaction is at least 2 kA/m.sup.2.

28. An electrolysis apparatus comprising a gas diffusion electrode as claimed in claim 15 as carbon dioxide depolarized cathode.

Description

EXAMPLES

[0058] The GDEs produced according to the following examples were used in oxygen electrolysis. A laboratory cell which consisted of an anode space and, separated off by an ion exchange membrane, a cathode space was used for this purpose. A KHCO.sub.3 solution having a concentration of 300 g/l was used in the anode space in which oxygen was produced at a commercial DSA with iridium-coated titanium electrode. The cathode space was separated from the anode space by a commercial cation exchange membrane from Asahi Glass, Type F133. Between GDE and the cation exchange membrane, there was an electrolyte gap in which an NaHCO.sub.3 solution having a concentration of 300 g/l was circulated by pumping. The GDE was supplied via a gas space with carbon dioxide whose concentration was greater than 99.5% by volume. Areas of anodes, membrane and gas diffusion electrodes were each 3 cm.sup.2. The temperature of the electrolytes was 25 C. The current density in the electrolysis was 4 kA/m.sup.2 in all experiments.

[0059] The GDEs were produced as follows: 3.5 kg of a powder mixture consisting of 7% by weight of PTFE powder, 93% by weight of silver powder (e.g. Type 331 from Ferro) were mixed in an Ika model A11 basic mill in such a way that the temperature of the powder mixture did not exceed 55 C. This was achieved by the mixing operation being interrupted and the powder mixture being cooled down. In total, mixing was carried out three times at a mixing time of 10 seconds. After mixing, the powder mixture was sieved through a sieve having a mesh opening of 1.0 mm. The sieved powder mixture was subsequently applied to an electrically conductive support element. The support element was a gauze composed of nickel having a wire thickness of 0.14 mm and a mesh opening of 0.5 mm. Application was carried out with the aid of a 1 mm thick template, with the powder being applied using a sieve having a mesh opening of 1.0 mm. Excess powder which projected above the thickness of the template was removed by means of a scraper. After removal of the template, the support with the applied powder mixture was pressed by means of a roller press using a pressing force of from 0.4 to 1.7 kN/cm. The gas diffusion electrode was taken from the roller press.

Example 1 Production of a Porous Silver Catalyst (According to the Invention)

[0060] 400 ml of a 0.1 molar AgNO.sub.3 solution (6.796 g of AgNO.sub.3) were admixed with 0.8 g of trisodium citrate. 400 ml of a 0.2 molar sodium borohydride (3.024 g of NaBH.sub.4) solution were added quickly while stirring to the first solution (about 15 s, Re>10000) and stirred for 1 hour. The precipitate was filtered off, washed with water and dried overnight at 50 C.

[0061] The powder was characterized by means of BET, laser light scattering and scanning electron microscopy.

[0062] Particle size is about 145 nm in diameter and the BET surface area is 2.23 m.sup.2/g (N.sub.2 adsorption).

Example 2 Production of a Less Porous Silver Catalyst

[0063] 400 ml of a 0.1 molar AgNO.sub.3 solution (6.796 g of AgNO.sub.3) are admixed with 0.8 g of trisodium citrate, 400 ml of a 0.2 molar sodium borohydride (3.024 g of NaBH.sub.4) solution are slowly added dropwise to the first solution (about 1 hour) while stirring and stirred for 1 hour. The precipitate was filtered off, washed with water and dried overnight at 50 C. The powder is characterized by means of BET, laser light scattering and scanning electron microscopy.

[0064] Particle size is about 290 nm in diameter and the BET surface area is 0.99 m.sup.2/g (N.sub.2 adsorption).

Production of GDE with Porous Silver

[0065] The GDE was produced by the dry process, with 93% by weight of silver powder as per example 1 and 2 and LCP-1 silver from Ames Goldsmith, and 7% by weight of PTFE from DYNEON TF2053 being mixed in an Ika model A11 basic mill and subsequently pressed by means of a roller press at a force of 0.5 kN/cm. The electrode was used in the above electrolysis cell and operated at 2 and 4 kA/m.sup.2. The Faraday efficiency for CO is shown in the table below.

TABLE-US-00001 Current density Current density Example BET m.sup.2/g 2 kA/m.sup.2 4 kA/m.sup.2 1 2.23 66 43 2 0.99 19 7 LCP-1 0.5-0.9 0 0

[0066] The examples show that both carbon dioxide GDEs produce carbon monoxide even at high current densities. However, it can be seen very clearly that the electrode containing more porous silver has a significantly higher selectivity to carbon monoxide than conventional silver. The selectivities at 2 kA/m.sup.2 are in an order of magnitude which is of great interest for industrial use. If LCP-1 silver particles, whose porosity is normally lower, are used, no CO rather only hydrogen is produced.

[0067] The BET measurements were carried out under the following conditions.

[0068] The physisorption of gases under cryogenic temperature conditions is used to determine the specific surface area (SSA) of compact finely divided or porous solids. Nitrogen is used as gas at 77K in the pressure range from 0.05 to 0.30 p/p0 (p0=saturation pressure of nitrogen at the measurement temperature) in order to determine the SSA of a sample. The amount of nitrogen which is physisorbed on the accessible surface area of the sample is measured in a static volumetric analyzer by introduction of a well-defined amount of nitrogen gas into the measurement cell containing the sample. At the same time, the pressure rise due to the introduced gas is recorded after the equilibrium state has been reached. The pressure rise (at equilibrium) is all the smaller, the larger the total area in the measurement cell, since the amount of nitrogen adsorbed on the surface cannot contribute to the pressure rise. The molar amount of nitrogen adsorbed on a sample enables the total area of the sample to be calculated by multiplication of the molar amount by the known adsorption cross section of the gas being adsorbed.

[0069] Before the adsorption measurement at 77 K, all desorbable molecules have to be vaporized from the sample surface. Thus, the sample was maintained under vacuum conditions for a number of hours at 200 C.

[0070] The measurement is then carried out in a manner analogous to the DIN ISO Standard 9277 using nitrogen of the purity class 5.0

[0071] Preparation instrument: SmartVacPrep (from Micrometrics) and gas adsorption analyzer: Gemini 2360.

[0072] The particle sizes were obtained by means of laser light scattering on a Malvern Mastersizer MS2000 Hydro MU instrument.