Means and Method for Reducing Carbon Dioxide to a Product

1. Field of the Invention

The present invention relates to electrochemical processes in general and, more particularly, to apparatus and the method for reducing carbon dioxide to provide a product.

2. Summary of the Invention

Apparatus for reducing carbon dioxide to the product includes a reduction cell which has a dual porosity cathode, a catholyte chamber having an inlet, a passageway through which passes an electrolyte, a dual porosity cathode separating the passageway from the catholyte chamber, an anolyte chamber has an inlet and an outlet. A porous anode with a hydrophobic barrier separates the passageway from the anolyte chamber. A source provides a d.c. voltage across the cathode and the anode. Water is provided to the inlet of the anolyte chamber, while an electrolyte is provided to the passageway. Carbon dioxide is provided to the inlet of the catholyte chamber so that the carbon dioxide is electrochemically reduced within the dual porosity cathode with the electrolyte and hydrogen ions so as to cause the reduction of the carbon dioxide to a product and to cause oxygen to be emitted from the outlet of the anode chamber. The product is removed from the electrolyte, present on the cathode side, after leaving the electrolytic cell.

The objects and advantages of the invention will be described more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

FIG. 1 is a partial schematic and partial cutaway drawing of apparatus for reducing carbon dioxide to a product constructed in accordance with one embodiment of the present invention.

FIGS. 2 and 3 are partial schematic and partial cutaway drawings of apparatus for reducing carbon dioxide to provide a product in accordance with other embodiments of the present invention.

The present invention electrochemically reduces carbon dioxide to valuable chemicals such as oxalate, formate, and formaldehyde. With an aqueous electrolyte solution the reduction rate is very slow. Better rates (higher current densities) are achieved using non-aqueous electrolyte systems for CO.sub.2 reduction to oxalate. The use of high catalytic surface area porous gas diffusion electrodes will maximize the three phase interface of the carbon dioxide, the catalyst and the electrolyte. This allows very high apparent current densities to be passed through the electrode for the reduction of the carbon dioxide to commercial chemicals at practical rates.

FIG. 1 shows a DSK type cell which consists of a pressure vessel 1 having an electrolyte passageway 5, a carbon dioxide chamber 9, a water/oxygen chamber 14. A dual porosity cathode 17 has a section 17A having coarse pores, which may be in the neighborhood of 20 to 60 microns in diameter, and a section 17B of fine pores having pore sizes of 2 to 5 microns in diameter. Cathode 17 as noted is made of porous material which may be titanium, stainless steel, nickel, raney nickel, cobalt, carbon, reticulated vitreous carbon and has specific catalysts introduced into the cathode such as lead or gold for formate production, or silicon for oxalate production. Indium phosphide, containing catalytic quantities of cobalt and lead, or mercury can be used in aqueous systems for formate production.

Carbon dioxide is provided to chamber 9 under pressure so there is no need for a hydrophobic barrier. An electrolyte source 23 provides either a non-aqueous or an aqueous electrolyte to passageway 5. A non-aqueous electrolyte for the oxalate production is dimethylformamide with a supporting electrolyte selected from the following: tetrabutylammonium perchlorate, tetrabutylammonium tetrafluoroborate, tetrabutylammonium hexafluorophosphate, tetraethylammonium perchlorate and tetraethylammonium tetrafluoroborate. An aqueous electrolyte may be 1M potassium chloride.

A single porosity anode 28 is adjacent passageway 5 so that there is an interaction within anode 28 resulting in the conversion of H.sub.2 O to oxygen which gives up the hydrogen ion for use in the formation of the product within the electrolyte. The electrolyte carries the product out. The product is then separated from the electrolyte and provided via line 32.

A d.c. voltage source 36 has its negative terminal connected to cathode 17 and its positive terminal connected to anode 28 and provides a direct current voltage across cathode 17, the electrolyte in passageway 5 and anode 28 to facilitate the transfer of electrons into the reaction area for the carbon dioxide.

FIG. 2 shows another embodiment of the dual porosity electrode concept. For convenience, those elements that are in FIG. 1 and which appear in FIG. 2, have the same numeric identification. There is shown a cell 40 having a dual porosity cathode 17 as one side with its coarse pore section 17A and its fine pore section 17B. Carbon dioxide is applied to section 17A at a predetermined pressure. There is a catholyte chamber 44 adjacent to cathode 17 the side walls of cell 40 and a separator 47 which has electrolyte provided through an inlet 49 into chamber 44 and a product leaving by an outlet 50 from chamber 44. There is another chamber formed by separator 47 and the side wall of cell 40 having an inlet 55 and an outlet 57 throug which the electrolyte is also provided to. There is within chamber 52 a porous anode 59 so that electrolyte entering inlet 55 passes through anode 59 and after the reaction passes as spent electrolyte from exit 57. The use of porous anode 59 eliminates mass transfer losses allowing cell 40 to operate at a lower voltage. D.C. voltage source 36 has its positive terminal connected to anode 59 and its negative terminal connected to cathode 17 to provide electrons for use in the reaction process.

With reference to FIG. 3, there is shown yet another form of the present invention in which there is a vessel 60, one end of the vessel 60 being a single porosity cathode 62 and a hydrophobic barrier 64 through which carbon dioxide gas passes, but not a liquid. The carbon dioxide passes through barrier 64 and through cathode 62 into a chamber 65 having an inlet 67 through which electrolyte enters and an outlet 69 from which a product exits. There is also present separator 47 and there is another chamber 52 which is the same as chamber 52 with its inlet 55 and outlet 57 as shown in FIG. 2. There is porous anode 59, the same as in FIG. 2. It should be noted that the electrolyte to be used in this embodiment is an aqueous electrolyte, such as 1M potassium chloride, which therefore requires the hydrophobic barrier 64.