DE2052955A1 - Porous oxygen electrodes for fuel cells - with acid electrolyte contg pfeiffer's complexes as catalysts - Google Patents

Abstract

Title electrodes contain mixtures of carbon black with chelate metal complexes derived from salicylaldehyde or its homologues and a diamine (Pfeiffer's complexes), pref. polymeric cpds. contg. a transition metal atom (pref. Co) as the central atom. They are highly resistant against hot H2SO4, and have high catalytic activity.

Description

Porous Oxygen Electrodes for Fuel Cells The invention relates to Cathodes for fuel cells with acidic electrolytes and air or oxygen as Oxidizing agent. Their goal is to make porous cathodes that last for a long time Are resistant to the attack of the acid over time and change their activity little.

For the reduction of oxygen at the cathode of fuel cells, i.e. the transfer of electrons from the electrode to the oxygen molecules, Catalysts are needed for the fuel cell's working conditions are stable for a long time. Cells with alkaline electrolytes are good catalysts known (e.g. silver); but an alkaline electrolyte becomes more carbon-containing when it is converted Fuels are consumed by the carbonate that forms on the anode. thats why it is advantageous to use acidic electrolytes (e.g. sulfuric acid or Phosphoric acid), as the CO formed from them escapes in gaseous form. However, are only a few catalysts are known that are sufficient to attack acidic electrolytes withstand long; primarily it is platinum and platinum metals (resp. their alloys), but their widespread use is opposed by their high price.

It is true that various organic compounds are favored by electrochemical Purposes have been suggested. These are, for example, polyanilines that are used in accumulators Can be used as an active mass, since it is reversibly oxidizable and reducible are (French patent 1,519,729 CNRS); organic dyes, which have been proposed because of their good adhesion to plastic surfaces are to firmly bond metal layers (catalysts) to such surfaces (Canadian Patent 720,318, Esso); Phthalocyanine compounds, some of which have shown some activity for oxygen reduction (U.S. Patent 3,410 727, Allis-Chalmers); Acetylacetonates as catalysts with a hydrophobic effect for fuel cell electrodes; however, only those compounds are suitable that Contain platinum metals as the central atom. The substances are in the finished electrode in more or less decomposed form (British patent 1 114 908, Union Carbide).

But all of these substances are either developed with a different objective been and catalytically inactive (polyanilines, dyes), or the activity, at least of the platinum metal-free compounds is low.

It has now been found that another group of substances the show a very high resistance to hot sulfuric acid, the electrochemical Catalyze oxygen reduction: these are metal complex compounds # (chelates), derived from salicylaldehyde and a diamine and referred to as Pfeiffer's for the chemist Connections are known. It must be emphasized that not those representatives of the class which contain platinum metals are particularly catalytically effective and of which one might assume that the platinum metal is in them unfolded catalyst properties known per se. True, such connections are known and show some catalytic activity; but are particularly active the cobalt compounds referred to by Pfeiffer as salcomins and those analogues, the transition metals with atomic numbers 22 to 28 of the periodic table of elements (Titanium to nickel) included.

The preparation of the compounds is known to the person skilled in the art (P. Pfeiffer et al., Liebigs Ann. 503 (1933) 84) and offers no experimental difficulties: Salicylaldehyde and a diamine (e.g. o-phenylenediamide) easily form a Schiff's Base from which metals capable of forming grains when cooked with salts (in alcoholic Solution) the chelate fails. Some complex compounds of this type are also formed when aldehyde, diamine and metal salt solution are combined at the same time.

It is known that oligomers can also be obtained in various ways or polymeric Schiff bases that are suitable for chelation, be it that one builds up polyamine chains, which with salicylaldehyde to polymeric ships' different bases react, chelating groups are adorned in side chains (Nikolaev et al., Vysokomol. Soed.

6 (1964) 1825 and 1829), or that bifunctional salicylaldehyde derivatives (such as 5,5'Methylene-bissalicylaldehyde) and diamines build up chain molecules where the chelating groups are in the molecular chain itself (Marvel and Tarköy, J. Am. Chem. Soc. 74 (1957) 6000 and 80 (1958) 832; Goodwin and Bailar, J.Am.Chem. Soc. 83 (1961) 2467). When metal salts are added, these substances form polymeric chelates. However, it is also possible to use monomers equipped with corresponding reactive groups To polymerize or polycondense metal chelates. The polymers obtained in this way If processed appropriately, they often have a higher catalytic activity than the corresponding ones Monomers and are even more resistant to attack by the Electrolytes.

When the - monomeric or polymeric - chelates are finely pulverized in pure form processed into porous electrodes -Methods for introducing electrically non-conductive Catalysts in porous, conductive support structures are known per se - so show they have a noticeable but very low activity for cathodic reduction of oxygen; a practical application cannot be thought of. In suitable Prepared form, however, they make good fuel cell catalysts.

It is known per se that the deposition of a catalyst occurs a high-surface carrier substance because of the enlargement of the active area often leads to an increase in activity. Surprisingly, however, is not alone the size of the carrier surface is decisive, but it could be determined that charcoal preparations of different origins with a comparable surface are too different lead active catalysts. In addition to the chemical nature of the coal surface (presence different adsorbates because of different activation processes) plays a role here especially the way in which the chelate is deposited plays an important role.

It was found that certain methods of preparation of the chelate precipitates result in particularly effective catalysts. It is that once the formation of the metal chelate in situ on a polymeric Schiff base soaked charcoal, on the other hand the adsorptive separation of the chelate on the way about the saturated solution. The chelates are in fact in many organic solvents only slightly soluble, so that a large amount of solvent would be necessary, to bring the chelate to be deposited into solution, from which it is by evaporation or Failures with a non-solvent could be brought onto the carbon. Brings but you put activated charcoal in a small amount of the chelated solution with chelate as a soil body, it becomes so long because of the low pressure of the solution of the sorbate Chelate adsorbed on the carbon and replenished in the appropriate amount from the soil, until the entire surface of the coal is covered with sorbate or there is no more sediment is. This creates a very uniform, thin, predominantly monomolecular one Covering the charcoal with chelate, so that the above-mentioned effect of the charcoal carrier can clearly appear on the chelate. Even when soaking the coal with solutions polymeric Schiff bases and subsequent reaction similarly thin layers can be achieved with metal salts. This can be proportionate work simply because the Schiff bases are in many organic solvents are easily soluble.

Claims (5)

Claims che 01. Porous oxygen electrodes for fuel cells with acidic electrolytes, characterized in that it acts as a catalyst for the reduction of oxygen Contain metal complexes that differ from salicylaldehyde or its homologues and derived from a diamine and which are known to chemists as Pfeiffer complexes, preferably polymeric compounds, the central atom of which is those of the series of transition metals contain. 2. Electrodes according to claim 1, characterized in that they den Catalyst intimately mixed with carbon in the form of activated charcoal or soot contain. 3. A method for producing electrodes according to claim 2, characterized characterized in that the catalyst is deposited adsorptively on the carbon, whereupon in a manner known per se by adding polymer powder and acting bodies are formed by pressure and heat as fuel cell electrodes suitable. 4. Process for the preparation of the catalyst mixture dir the process according to claim 3, characterized in that the carbon is in the form of activated carbon or soot in a saturated solution of the metal complex, which also contains the metal complex as a soil body, enters, whereupon it is stirred until a smooth weight between the complex adsorbed on the carbon and the complex in solution has set. 5. Process for the preparation of the catalyst mixture for the process according to claim 3, characterized in that the carbon is in the form of activated carbon or soot soaked with a solution of a polymeric Schiff base and then nit the solution of a metal salt is treated to form a Pfeiffer's Complex connection.