DE69033409T2 - Electrolytic cell and method of use - Google Patents

Abstract

The present invention discloses an improved solid polymer electrolysis cell (2) for the reduction of carbon dioxide. The improvement being the use of a cathode (6) having a metal phthalocyanine catalyst which results in the suppression of the formation of hydrogen during the reduction process and the subsequent improved conversion efficiency for carbon dioxide.

Description

Technical area

The technical field to which this invention relates is electrolysis cells, in particular electrolysis cells with solid polymer electrolyte membrane.

Background of the invention

The electrochemical reduction of carbon dioxide to produce organic compounds using an electrolytic cell has been known for some time. Such reduction has been carried out in conventional electrolytic cells having an anode, a cathode and an electrolyte. The cells are typically operated by passing an electric current through the anode and cathode at the same time that a fuel is brought into contact with the catalyst at the anode and a carbon dioxide-containing fluid is in contact with the catalyst at the cathode. The typical fuel contains hydrogen and is either hydrogen gas or water. One such process is described in U.S. Patent No. 4,609,441 for the production of methanol, while a second for the production of hydrocarbons is taught in the article entitled: Ambient Temperature Gas Phase CO₂ Reduction to Hydrocarbons at Solid Polymer Electrolyte Cells, J. Electrochem. Soc., Electrochemical Society and Technology, June 1988, pp. 1470-1471.

The cathode used according to this paper was Cu, Ni or Rh as a coating on NaN 117. Pt was used as an anode.

The problem associated with operating these devices is that it has not been possible to design an electrolytic cell having adequate conversion efficiency to be of any real commercial value. This is demonstrated in the article cited above, where the conversion rate of carbon dioxide to hydrocarbons is less than about 2%.

In "Chemical Abstract, Vol. 108, No. 26, June 27, 1988, p. 440, Abstract No. 228382k, Columbus, Ohio, US" the electrochemical reduction of carbon dioxide to methane and ethane at copper/Nafion electrodes in solid polymer electrolyte cells is disclosed. The electrodes were prepared by an electroless plating process and provided a Faradaic efficiency of 20% as a steady state value.

The present invention is directed to improving the conversion performance of electrolysis cells.

Disclosure of the invention

The present invention relates to an improved electrolytic cell as defined in claim 1.

The foregoing and other features and advantages of the present invention will become more apparent from the following description and drawings.

Short description of the drawings

Fig. 1 shows a sectional view of an embodiment of an electrolytic cell of the present invention.

Best mode for carrying out the invention

Fig. 1 shows a typical electrolytic cell 2 of the present invention, containing an anode 4, an anode compartment 5, a cathode 6, a cathode compartment 8 and a solid polymer electrolyte 10 and current collectors 12 and 14. A typical electrolytic cell is described in co-assigned U.S. Patent No. 3,992,271.

The anodes useful in these cells are made of conventional materials such as platinum, ruthenium or iridium. In addition, mixtures or alloys of these and other materials distributed on a high surface area support may also be used. Conventional anodes that are particularly useful are described in co-assigned U.S. Patent No. 4,294,608 and in the aforementioned U.S. Patent No. 3,992,271. The catalyst on the anode should have high reactivity for the half-cell reaction

2H2O ? 4H+4e- + O&sub2; (1)

or

H&sub2; ? 2H+ + 2e&supmin;

be able.

These anodes are attached to the solid polymer electrolyte using conventional techniques. This is generally achieved by the process of bringing the anode into contact with a surface of the electrolyte membrane and causing the anode to bond to it by the application of pressure at elevated temperature.

The electrolyte may be any of the conventional solid polymer electrolytes useful in fuel cells or electrolysis cells and capable of transporting positive ions (preferably H+) from the anode to the cathode. One type is a cation exchange membrane in protonated form such as Nation (registered trademark, available from DuPont Corporation). Other possible electrolytes may be perfluorocarboxylic acid polymers available from Asahi Glass and perfluorosulfonic acid polymers available from Dow Chemical. These and other solid polymer electrolyte materials are well known to those skilled in the art and need not be detailed here.

The improvement over the prior art electrolysis cells includes the selection of a primary cathode material and the introduction of a carbon dioxide reducing secondary cathode into the cell.

Primary cathode

During the reduction of carbon dioxide in the electrolytic cell, many reactions can take place at the cathode, resulting in a number of possible compounds being formed.

The most predominant reaction is the reduction of carbon dioxide to formic acid as shown below

CO&sub2; + 2H+ + 2e&supmin; ? HCOOH (2)

However, several other reactions can take place, such as the production of methanol and formaldehyde.

CO&sub2; (g) + 4H+ + 4e- → HCHO + H2 O (3)

CO&sub2; (g) + 6H+ + 6e&supmin; ? CH3 OH + H2 O (4)

Meanwhile, subsequent reactions can produce other organic compounds such as methanol or methane.

HCOOH + 2H+ + 2e&supmin; ? HCHO + H2 O (5)

HCOOH + 4H+ + 4e&supmin; ? CH3 OH + H2 O (6)

HCHO + 4H+ + 4e&supmin; ? CH&sub4; + H2 O (7)

Depending on the current density at which the cell is operated and other operating parameters of the electrolytic cell including the type and concentration of the reactants, one or more of these compounds will be produced at the cathode.

However, there is a reaction that will slightly reduce the conversion rate of carbon dioxide at the cathode if that reaction is allowed to take place. This reaction is the formation of H2 gas as shown in Equation 8

2H+ + 2e&supmin; ? H2 (g) (8)

is shown.

It is believed that by using a cathode with a greater hydrogen overpotential than platinum, which will suppress the formation of hydrogen gas and thereby increase the amount of hydrogen ions available to react with the carbon dioxide, this undesirable competitive reaction can be reduced.

It is a teaching of this invention that the primary cathode 6 should comprise a material that has a tendency to reduce carbon dioxide as well as having a greater hydrogen overvoltage than platinum, namely metal porphyrins and metal phthalocyanines. Typical metal porphyrins are aluminum and zinc porphyrins. The most preferred materials are the metal phthalocyanines. Any metal phthalocyanine may be used, with the preferred material being nickel phthalocyanine. Other representative metal phthalocyanines are listed in Table I below:

TABLE I

Cobalt phthalocyanine

Iron phthalocyanine

Copper phthalocyanine

These phthalocyanines probably have the formula

where M can be any metal ion, preferably cobalt, iron, nickel or copper. It is also possible to form the cathode using a mixture of these materials or to mix them with other catalytic materials. However, it should be borne in mind that other catalytic materials may prove detrimental to the conversion performance, particularly if they have a low hydrogen overpotential, as it may promote the formation of hydrogen gas.

The cathode containing these materials is manufactured using conventional techniques and is applied directly to the electrolyte membrane in a conventional manner, typically by the application of heat and pressure. Typically this means mixing the catalytic material with a binder such as polytetrafluoroethylene len or other inert material which does not adversely affect the reactivity of the cathode. Generally, the mixture will be in a proportion of from about 5% to about 50% by weight, with a preferred range of from about 15% to about 20% by weight, of the catalytic material, but the actual amount required will vary depending on the catalytic material chosen.

Secondary cathode

In addition to the primary cathode, a secondary cathode is also introduced into the cell. This secondary cathode is in the form of a separate structure as shown in Fig. 1 as 18. In either arrangement, the secondary cathode must be in electrical contact with the primary cathode and in physical contact with the carbon dioxide and hydrogen ions. The secondary cathode is positioned in the flow path of the carbon dioxide as shown in Fig. 1 and is preferably supported on a plurality of fine wire screens shown in Fig. 1 as 18. The secondary cathode comprises a catalytic coating material which again has a greater hydrogen overvoltage than platinum and a tendency to reduce carbon dioxide in the presence of hydrogen ions.

These two characteristics are important in selecting a suitable material for two reasons. First, as discussed for the primary cathode electrode material, it is important for the enhanced reduction of carbon dioxide that the formation of hydrogen gas is suppressed as shown in Equation 8. The second characteristic, its propensity to reduce carbon dioxide in the presence of water ions, leads to an increase in the number of available reaction sites for this reduction to take place.

Catalytic materials that may be useful in making such a secondary cathode may be inorganic metals such as ruthenium, indium, iridium, copper, or mixtures of metals such as steel or stainless steel, all of which satisfy both requirements for a secondary catalyst.

Organic materials can also be used, just like those in the primary cathode. The organic materials of particular importance are the macromolecules, metal porphyrins or metal phthalocyanines discussed above for use in the primary cathode.

It is believed that the secondary cathode provides a significant increase in the number of active sites for the reduction of carbon dioxide to take place, thereby leading to a dramatic increase in the efficiency of the cell. The efficiency of the test cell described below increased from about 60% to over 90% with the addition of this secondary cathode.

The method of manufacture for this secondary cathode is, when it is in the form of a metal or metal composition, as a fine mesh screen. This allows the cathode to have a very high surface area and to be easily inserted into the cathode region of the cell. In this form, the secondary cathode is formed from one or more of these screens.

If the material is formed of an organic material, it can be compressed to form a cathode, or it can be mixed with a binder such as polytetrafluoroethylene and then pressed to form the cathode, as is done for the primary cathode. Or it can be deposited on a substrate. The substrate can be formed of an inert material, or it can be formed of catalytic material. Preferably, the support will also have a greater hydrogen overvoltage than platinum so that it does not contribute to the formation of hydrogen gas. The preferred way is to plate or deposit the material on a support structure such as a fine wire screen. Such is the case with the preferred secondary cathode structure in which indium is deposited on a fine stainless steel wire screen.

The electrolytic cells operate when a potential is created between the anode and cathode. The magnitude of the potential must be such that hydrogen ions are created at the anode and carbon dioxide is reduced at the cathode. The actual voltage requirements vary depending on a number of variables. The type of catalysts used in the anode and cathode are important to the voltage requirements, as is the type of anolyte or catholyte used. For example, an anolyte composed of hydrogen gas would have lower voltage requirements than an anolyte composed of water. In addition, the arrangement and construction of the actual cell components, i.e. flow fields, can change the voltage requirements. Typically, these electrical requirements will be in the range of about 2V to about 5V.

The potential can be generated by any conventional means such as common electrical sources, i.e. batteries or fuel cells. In the reduction of carbon dioxide, the anode will be positively charged while the cathode will be negatively charged. In addition to ionizing the hydrogen in the anolyte and reducing the carbon dioxide in the catholyte, the potential on the solid polymer electrolyte drives the hydrogen ions through the electrolyte from the anode to the cathode so that they are available to react with the carbon dioxide.

Operation of the electrolytic cell during the reduction of carbon dioxide is conventional. Typically, operation involves the introduction of hydrogen or water into the anode side of the cell and carbon dioxide into the cathode side of the cell. The hydrogen gas may be introduced at ambient pressure, but it is preferred that it be introduced at pressures greater than 34.4 N cm⁻² (50 psig), with a preferred range of 551.2 N cm⁻² (800 psig) to 620.1 N cm⁻² (900 psig). Water, however, may be introduced at ambient pressure or above, with the preferred range being 151.2 N cm⁻² (800 psig) to 620.1 N cm⁻² (900 psig). The carbon dioxide may be introduced as a gas mixture, as a liquid, or dissolved in an aqueous solution such as lithium carbonate, or in another form that does not interfere with the function of the solid polymer electrolyte membrane (i.e. too cold or a non-aqueous solution). At the same time that the materials are introduced into the cell, an electric current is passed between the anode and the cathode sufficient to cause the dissociation of the hydrogen or water and to cause the hydrogen ions to be transported through the electrolyte to the cathode, where the carbon dioxide is reduced to an organic compound in the presence of the primary and secondary cathodes.

An example of an electrolytic cell of the present invention was used to reduce carbon dioxide and is described below.

Example

An electrolytic cell for the reduction of carbon dioxide was prepared having a 46 cm² (0.05 ft²) cathode of nickel phthalocyanine and Teflon in a mixture of 85 wt% to 15 wt% pressed onto the electrolyte. In addition, a secondary cathode in the form of 6 to 40 mesh screens of 316 stainless steel electroplated with indium was used. In addition, an indium plate was tack welded to the fluid distribution plate formed by the collector plate to promote fluid agitation in the carbon dioxide stream and to improve contact with the two cathodes.

A solution of argon in equilibrium with 0.1 molar lithium carbonate was passed over the anode at a pressure of 206.7 N cm⁻² (300 psig) at a flow rate of about 200 to 500 cm²/min. While a solution of carbon dioxide in equilibrium with 0.1 molar lithium carbonate was passed over the anode at a pressure of 223.9 N cm⁻² (325 psig) and a flow rate of about 200 to 500 cm²/min. The cell was operated at a current density of 53.8 mA x cm² (40 A/ft²) for 42 minutes at a temperature between 23.9ºC (75ºF) and 37.8ºC (100ºF).

Samples of the recirculated lithium carbonate solution were taken from the cathode area of the cell and analyzed for organic liquid reactants using an ion chromatograph. The results showed the presence of 2103 parts per million of formic acid, which, after a mass balance, resulted in a conversion efficiency for the carbon dioxide to formic acid of 90%.

Claims (10)

1. Electrolysis cell (2) for the reduction of carbon dioxide with an anode (4), a cathode (6, 18) and a solid polymer electrolyte (10), characterized in that the cathode has: (a) a carbon dioxide reducing primary cathode (6) which is formed as a layer on a surface of the solid polymer electrolyte (10) and contains at least one metal phthalocyanine or metal porphyrin as a catalytic material, (b) and a carbon dioxide reducing secondary cathode (18) in electrical contact with the primary cathode (6), the secondary cathode (18) being in the form of one or more grids coated with a carbon dioxide reducing catalyst having a greater hydrogen overvoltage than platinum. 2. Electrolytic cell according to claim 1, wherein the grid(s) comprises stainless steel, copper, brass, niobium, zirconium or titanium. 3. Electrolysis cell according to claim 1 or 2, wherein the catalyst coating is a metal. 4. Electrolytic cell according to one of claims 1 to 3, wherein the catalyst coating is formed from a material selected from the group consisting of tin, lead, copper, Zinc, cadmium, gallium, silver, gold, indium, iron, tungsten, molybdenum and carbon. 5. Electrolytic cell according to claim 4, wherein the catalyst coating is indium. 6. Electrolysis cell according to claim 1 or 2, wherein the catalyst coating is a metal phthalocyanine or a metal porphyrin. 7. Electrolytic cell according to claim 6, wherein the metal phthalocyanine is selected from the group consisting of iron, copper, nickel and cobalt phthalocyanine. 8. Electrolytic cell according to claim 7, wherein the metal phthalocyanine is nickel phthalocyanine. 9. Electrolytic cell according to one of claims 1 to 8, wherein the primary cathode is selected from the group consisting of iron, copper, nickel and cobalt phthalocyanine. 10. Electrolytic cell according to claim 9, wherein the metal phthalocyanine is nickel phthalocyanine.