Electrolysis cell

Abstract: The present invention discloses an improved solid polymer electrolysis cell for the reduction of carbon dioxide. The improvement being the use of a cathode 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 Field The technical field to which this invention pertains is electrolysis cells for the reduction of carbon dioxide using a solid polymer electrolyte (SPE). Background of the Invention The electrochemical reduction of carbon dioxide to produce organic compounds utilizing an electrolysis cell has been known for some time. Such reduction has been carried out in conventional electrolysis cells having an anode, a cathode and an electrolyte. Typically the cells are operated by passing an electric current through the anode and cathode at the same time that an anolyte fuel is brought into contact with the catalyst on the anode and a carbon dioxide containing catholyte 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 4,609,441 for the production of methanol, while a second is taught for the production of hydrocarbons in the article entitled: Ambient Temperature Gas Phase CO 2 Reduction to Hydrocarbons at Solid Polymer Electrolyte Cells, J.Electrochem. Soc.: Electrochemical Society and Technology, June 1988 p 1470-1471). This document describes electrochemical reduction of CO 2 to hydrocarbons with one or two C atoms at Cu electrodes supported on SPE membrane, preferably Nafion. It is said that Cu is electrocatalytically active for promoting high rate CO 2 reduction in CO 2 saturated aqueous solutions. Said document is silent with respect to phthalocyanines. A chronic problem associated with operating these devices is that it has not been possible to devise an electrolysis cell which has an 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 percent. Catalysis Letters, vol. 1, 1988, J. C. Baltzer AG, Basel, Switzerland, pages 73 to 79 describes an electrolysis cell and method for reduction of CO 2 to hydrocarbon products including CH 3 OH at a Cu cathode in contact with a SPE consisting of Nafion. The CO 2 is fed to the cathode in the gas phase while the counter electrode reactant is a solution of H 2 SO 4 . It is mentioned that Cu alone is completely inactive for hydrogenation whereas Cu alloy catalysts have shown activity for the hydrogenation. D1 is silent regarding metal phthalocyanines. US-A-4 595 465 discloses a device for the reduction of CO 2 to oxalates. It comprises two photosystems and three chambers separated by two membranes consisting of Nafion with photosensitizers deposited thereon. Among a lot of other catalysts metal phthalocyanines may be used as such photosensitizers. The electrodes are separated from said membranes and are immersed in fluidic electrolytes. Not any material for said electrodes is mentioned in US-A-4 595 465. According to J. Electrochem. Soc., vol. 131, No. 7, 1984, pages 1511 to 1514, metal phthalocyanines deposited on C electrodes are found to catalyze the electroreduction of CO 2 to HCOOH in aqueous acid solutions saturated with CO 2 by electrolysis. At pH above 5 HCOOH is formed; CH 3 OH is also produced at lower pH values. A glassy C rod is polished and cleaned prior to depositing the catalyst, namely metal phthalocyanines. A thin layer of ca. 10 µ g of metal phthalocyanines is deposited on the C surface. Only Co phthalocyanines and Ni phthalocyanines are used. It is emphasized that graphite and glassy C seem to be specific in their ability to utilize phthalocyanines as catalysts for CO 2 reduction. J. Am. Chem. Soc. 1984, 106, pages 5033 to 5034 discloses the electrocatalytic reduction of aqueous solutions of CO 2 to CO using Co phthalocyanine as catalyst. The Co phthalocyanine is deposited on pyrolytic graphite or C by adsorption in a monolayer coverage. US-A-4 668 349 discloses the electrocatalytic reduction of aqueous solutions of CO 2 to CO using transition metal complexes with square planar geometry, e. g. metal phthalocyanines. Preferably Co phthalocyanine is adsorbed on a glassy C electrode, polished with alumina and sonicate. Documents J. Am. Chem. Soc. 99, 1 1977, pages 286 to 288, Römpps Chemie-Lexikon, 8th edition, Stuttgart 1983, pages 1608 to 1610, and Römpps Chemie-Lexikon, 8th edition, Stuttgart 1985, page 3200, catchword "Phthalocyanin-Farbstoffe" deal with the properties of metal phthalocyanines and, respectively, semiconductors, but use of said metal phthalocyanines in electrolysis cells or a similar use is not mentioned there. In particular, J. Am. Chem. Soc. 99, 1 1977, pages 286 to 288 discloses that metal phthalocyanines have a very low electrical conductivity. Conductivity increases dramatically upon oxidation with iodine of the metal phthalocyanines complexes. This document is silent regarding the ionic conductivity of metal phthalocyanines. Römpps Chemie-Lexikon, 8th edition, Stuttgart 1985, pages 3200, catchword "Phthalocyanin-Farbstoffe" discloses that metal phthalocyanines, i.e. phthalocyanine colouring agents, by partial oxidation with iodine acquire electric conductivity comparable to metals. It is further disclosed that these complexes have semiconductor properties. Römpps Chemie-Lexikon, 8th edition, Stuttgart 1983, pages 1608 to 1610 discloses that organic crystals like phthalocyanines are semiconductors. It is further disclosed therein, that in semiconductors electrical conductivity increases with temperature. Semiconductors may be ionic conductors or electronic conductors. Disclosure of the Invention The present invention is directed towards an electrolysis cell being operable to reduce carbon dioxide to a product consisting essentially of methanol and/or formic acid, comprising an anode, a cathode, and, at the cathode side of said electrolysis cell, a material having catalytic effect containing at least one metal phthalocyanine, characterized in that a solid polymer electrolyte capable of transporting positive ions is provided; and that said material having catalytic effect constitutes simultaneously the cathode, said cathode being formed of (a) at least one metal phthalocyanine, or (b) a mixture of at least one metal phthalocyanine and at least one other catalytic or non-catalytic material. The foregoing and other features and advantages of the present invention will become more apparent from the following description and drawings. Brief Description of the Drawings The Figure is a cross-sectional view of an electrolysis cell of the present invention. Best Mode for Carrying Out the Invention Conventional electrolysis cell structures may be used in the practice of this invention. One such conventional configuration is shown in the Figure which contains an electrolysis cell 2 having an anode 4, an anode chamber 6, a cathode 8 and a cathode chamber 10. The anode 4 and the cathode 8 are in electrical contact with a solid polymer electrolyte 12. In addition each chamber contains electrically conductive current distributors 14 as well as optional fluid distribution fields 16 shown in the anode chamber 6 (one may also be present in the cathode chamber as well if desired). Also present are inlet and outlet ports for the introduction and exhaustion of both the anolyte and the catholyte materials and the resulting products of the electrolysis reaction as well as a source of electrical current to the anode and cathode (for simplicity sake these structures are not depicted). A typical electrolysis cell is described in commonly assigned U.S. Patent 3,992,271. The anodes useful in these cells are conventional and will contain conventional catalytic materials and should be formed of conventional materials, such as platinum, ruthenium or iridium, using conventional techniques. In addition, mixtures and alloys of these and other materials dispersed on a high surface area support may also be used. Conventional anodes which are particularly useful are described in commonly assigned U.S. Patent 4,294,608 and the above mentioned U.S. Patent 3,992,271. The catalyst on the anode should be capable of high reactivity for the half cell reaction 2H 2 O → 4H + + 4e - + O 2 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 proton form such as Nafion (registered trade mark, 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 set forth in detail here. The improvement comprises the selection of the cathode material. It is believed that the presence of metal phthalocyanines at the cathode will improve the conversion efficiency of carbon dioxide in the presence of hydrogen ions to organic compounds. The most prevalent reaction is the reduction of carbon dioxide to formic acid set forth below CO 2 + 2H + + 2e - → HCOOH However, several other reactions may also be enhanced through the use of this cathode such as production of methanol, formaldehyde, glycolic acid, and methane. One or more of these materials will be generated at the cathode depending on the current density at which the cell is operated and other operating parameters of the electrolysis cell including the reactants. In the present invention, essentially formic acid and/or methanol are formed. Although it is believed that any metal phthalocyanine may be used in this invention the preferred materials are copper, iron, nickel and cobalt phthalocyanine with the most preferred being nickel phthalocyanine. The metal phthalocyanines should have a formula as set forth below wherein M is a metal ion such as copper, iron, nickel or, cobalt. The cathode containing the metal phthalocyanine may be formed using conventional techniques and can be applied to the electrolyte membrane in the conventional manner using heat and pressure. The resulting electrolysis cell should give surprisingly high efficiencies for the conversion of carbon dioxide to organic compounds, essentially formic acid and/or methanol. These efficiencies for the conversion of carbon dioxide to formic acid are likely to be in excess of 30 percent when the cell is operated using water as the fuel. It is believed that the improved conversion rate results from the ability of the metal phthalocyanines to suppress the formation of hydrogen gas via the reaction 2H + + 2e → H 2 (g) This is important as free hydrogen ions are necessary for the reduction of the carbon dioxide as may be seen in equation 2. It is believed that this competing reaction (the production of hydrogen gas) is enhanced by those cathode materials having a low hydrogen overvoltage, while the metal phthalocyanines have a high hydrogen overvoltage. (a high hydrogen overvoltage would be one greater than platinum.) The cathode may be formed of a single metal phthalocyanine or a mixture of metal phthalocyanines. It may even be made using other catalytic materials or noncatalytic materials mixed in with the phthalocyanines. However, these additional catalytic materials (particularly if they have a low hydrogen overvoltage) may enhance the formation of hydrogen gas and therefore reduce the conversion of carbon dioxide. This increase in the production of hydrogen gas would result in the reduced efficiency of carbon dioxide reduction. The catalytic loading levels for these cathodes would likely be from about 0.5 milligrams/cm 2 to about 10 milligrams/cm 2 of phthalocyanine. The method of reducing carbon dioxide using the present invention is as follows. The hydrogen containing anolyte is introduced into the anode chamber via an inlet source (not depicted). The anolyte comes in contact with the catalytic anode which is electrically charged. The anolyte undergoes an electrical reaction thereby producing free hydrogen ions. The free hydrogen ions are then transported across the solid polymer electrolyte membrane where they come in contact with the catalytic cathode. At the cathode side of the electrolysis cell a carbon dioxide containing catholyte is introduced into the cathode chamber and is brought into contact with the cathode. At the same time an electrical charge is being passed through the cathode. At the cathode where the hydrogen ions and the carbon dioxide contact the catalytic cathode the desired reaction takes place producing one or the other or a mixture of the products set forth in the specification. Although the cell may be operated at ambient pressure it would be preferred that the anolyte and the catholyte be introduced and maintained at an elevated pressure. Most preferably the pressure should be greater than 68.9 N cm -2 (100 psi) and even more preferably above 344.5 N cm -2 (500 psi). The preferred range of pressures would be between about 137.8 N cm -2 (200 psi) to about 689 N cm -2 (1000 psi) with about 413.4 to about 620.1 N cm -2 (600 to about 900 psi) being the optimum range. After the reactions have taken place at the anode and the cathode the reaction products and any residual anolyte and catholyte are passed out of the cathode and anode chambers respectively through outlet ports in each chamber (not shown). It is believed that the higher pressures improve the contact between the carbon dioxide and the cathode thereby increasing the chance for a favorable reaction. The present invention should make the use of these electrolysis devices practical for a number of commercial applications. The most useful of these applications may be found in closed loop environments such as spacecraft, space stations, or undersea habitats. In such environments animals, humans or machinery consume oxygen and produce carbon dioxide. The current invention permits the conversion of such carbon dioxide to an organic fuel i.e., formic acid. The formic acid may then be used to power a fuel cell to produce the electricity to power the electrolysis cell. In addition, it is intended as a primary use that the electrolysis cell be used with water as the fuel. This would permit the electrolytic decomposition of water to form oxygen which could then be consumed by the animals, man, or machinery while supplying the hydrogen ions for the carbon dioxide reduction.

Claims: An electrolysis cell (2) being operable to reduce carbon dioxide to a product consisting essentially of methanol and/or formic acid, comprising an anode (4), a cathode (8), and, at the cathode side of said electrolysis cell (2), a material having catalytic effect containing at least one metal phthalocyanine, characterized in that a solid polymer electrolyte (12) capable of transporting positive ions is provided; and that said material having catalytic effect constitutes simultaneously the cathode (8), said cathode being formed of (a) at least one metal phthalocyanine, or (b) a mixture of at least one metal phthalocyanine and at least one other catalytic or non-catalytic material. The electrolysis cell (2) of claim 1 wherein said at least one metal phthalocyanine is selected from the group consisting of iron, copper, nickel or cobalt phthalocyanine or mixtures thereof. The electrolysis cell (2) of claim 1 wherein said at least one metal phthalocyanine is nickel phthalocyanine.

Inventors: MOLTER TRENT M, MOLTER, TRENT M.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1990
Main classification: C25B3
IPC codes: C25B11, C25B3, C25B9