WO1998018171A1 - Inert cathode for selective reduction of oxygen and a method for the production thereof - Google Patents

Abstract

Inert cathodes, provided with a catalytically effective thin surface coating, enabling oxygen reduction are suitable for fuel cells. However, in the presence of methanol as a combustible, this coating must exhibit selective properties which cannot be obtained by using semiconductive cluster materials made of transition metal chalkogenides in the form of powder or a fine coating. The inventive inert cathodes possess the desired catalytic and selective properties. Their thin surface coating comprises colloidally dispersed homogenously distributed clusters and its production requires only a minimal amount of material in the nanogram range for a low coverage rate with a highly selective and catalytic degree of action. The coating is obtained by immersion in a colloid containing the cluster mixture and a stabilizer in a solvent.The solvent and stabilizer are removed by temperature treatment after application of the colloid.

Description

Inert cathode for selective oxygen reduction and process for its production

description

The invention relates to an inert cathode for the selective reduction of oxygen in an acidic medium by multi-electron transfer for electrochemical energy conversion in a methanol fuel cell with a thin surface coating of a selectively active catalyst material, based on semiconducting clusters of at least one transition metal and a chalcogen, and a method for their manufacture.

With the emission-free fuel cell technology, electricity can be generated efficiently and in an environmentally friendly manner without having to go through heat production. Fuel cells are electrochemical cells that continuously convert the chemical energy change of a fuel oxidation reaction into electrical energy. The multi-electron transfer forms the physical basis for the energy conversion. At the anode, fuel molecules are oxidized with the release of electrons. The ions formed on the anode and the cathode migrate to the electrodes in a closed circuit in the electrolyte and combine there to form water and carbon dioxide. The use of an electrolytic acid provides natural protection against contaminating carbonate formation.

State of the art

The use of simple methanol as a harmless fuel with a high energy density enables the fuel cell to be operated in the low temperature range between 80 ° C and 120 ° C. The one taking place However, methanol crossover is not without problems. The most frequently used catalyst material for the cathode so far is platinum, which has a broad catalytic capacity, even with respect to methanol, but also has the risk of poisoning the platinum electrode. Because of this and because of the high material costs for platinum, new, selectively effective catalyst materials were increasingly sought. Here one came across semiconducting cluster compounds of transition metals embedded in a matrix of chalcogen atoms. These cluster compounds with a so-called "chevrel phase structure" show the selective effect, ie that they neither oxidize nor reduce methanol, but only reduce oxygen. This saves the use of complex and unreliable membranes to keep methanol away from the cathode.

DE 36 24 054 A1 describes the description of an inert electrode made of a semiconducting molybdenum cluster chalcogenide with catalytic activity for oxygen reduction to water in fuel cells. This mixed cluster material is polycrystalline and, in the ground powder state, has a grain size of between 1 μm and 5 μm which determines the layer thickness on the electrode, so that a coarsely disperse material system is present after mixing with the solvent. After applying a small amount of liquid and then evaporating the solvent, the cluster distribution on the electrode surface is not homogeneous, which can lead to a deterioration in the selective and catalytic properties of the electrode.

DE 38 02 236 A1 shows how a powder or a thin layer or film is produced from a mixture of substances to be reacted (metal carbonyl, chalcogen-containing compound and inert organic solvent) and for an inert electrode with catalytic activity in fuel cells can be used. This metal chalcogenide is also polycrystalline and leads to a coarsely dispersed system in the solvent. It has coagulated clusters with a diameter in the μm range, which due to their size do not lead to a homogeneous distribution - Despite the method of spray pyrolytic deposition disclosed here, which, however, uses a lot of material, it can lead to a surface coating on the electrode and can cause a locally different catalytic and selective effectiveness. The layer thickness of the coating is limited downwards by the cluster size.

The production of such catalyst material is complex and only possible in the high temperature range around 1100 ° C. Therefore, other methods were sought to produce materials with similar properties.

The prior art on which the invention is based is based on the general statements made above by the article "Novel Low-Temperature Synthesis of Semiconducting Transition Metal Chalcogenide Electrocatalyst for Multielectron Charge Transfer: Molecular Oxygen Reduction" by 0. Solorza-Feria et al., Electrochimica Acta, Vol.39, No. 11/12 (1994), pp. 1647-1653. This review shows that semiconducting cluster material of the formula (Ruι _-x Mo _x ) ySeO _z with 0.02 <x <0.04, 1 <y <3 and z ≡ 2y from a mixture of the metal carbonyls Ru3 (COι ₂ ) and Mo (CO) β and selenium can be obtained as a solvent by a wet-chemical-organic low-temperature synthesis in xylene Mixed clusters are based on two transition metals and a chalcogen and can be made available in the form of powder or thin layers (<0.5 μm) for the surface coating of the inert cathode Apparent forms have a platinum-like electrocatalytic activity with regard to oxygen reduction in the acidic medium and react selectively. However, these properties are limited by the packing density of the individual clusters in the coating, which is due to the appearance as a powder with coagulated clusters or as a thin layer with closely adjacent clusters and can lead to a mutual impediment of the clusters in their effect.

The problem on which the invention is based is therefore to be seen in the selective selection of an inert cathode with such a surface coating to provide effective catalyst material which brings about a further improvement in the selectivity and the catalysis of the oxygen reduction in acidic methanolic solution compared to the materials in the known manifestations. In addition, a method for producing such a cathode should be simple and inexpensive to carry out.

This object is achieved according to the invention in that the thin surface coating has a degree of coverage from individual clusters in a colloidally disperse homogeneous distribution, which is generated by a small amount of material in the ng range per cm ^{2 of} effective cathode area.

Such a distribution of the individual clusters enables them to develop their full catalytic and selective activity without interfering with one another. This activation of each individual cluster achieves a high level of effectiveness. The catalytic ability exceeds that of the materials in the above-mentioned manifestations. The colloidal dispersion according to the invention, ie. very finely divided arrangement of the individual clusters with a particle diameter in the nm range (for example 3 to 4 nm) in a homogeneous appearance effectively protects the cathode against catalyst poisoning and thus against a decrease in activity. Elaborate membranes are not required. Together with this improved selectivity in oxygen reduction and the associated long-term stability, the cathode according to the invention, with its thin surface coating, which only requires a layer thickness of, for example, 6 to 7 nm, has an advantage over a much thicker coating with a powder or a thin layer. In addition, the extensive distribution of the individual clusters, which leads to the low degree of coverage of the cathode surface, means that the material used is correspondingly low and in the ng range (for example 3.1 ng). In addition, the catalyst material is very temperature stable, so that heat treatments can be carried out.

In anticipation of the following explanations, it should be noted at this point that the advantageous properties mentioned of the invention according to the cathode is made possible by an appearance of the catalyst material itself as a colloidal solution which is easy to synthesize. A customized product for the high electrocatalytic activity can thus be provided.

The basis of the catalyst material with a colloidally disperse cluster distribution is formed by semiconducting transition metals, in particular molybdenum (Mo), tungsten (W), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh) and iridium (Ir), as well as the chalcogens Sulfur (S), selenium (Se), especially red selenium, and tellurium (Te). Binary clusters and ternary mixed clusters can be generated from these elements with the participation of oxygen (0).

According to one embodiment of the inert cathode according to the invention, it is advantageous if the transition metal is ruthenium (Ru) and the chalcogen is sulfur (S) or selenium (Se) and the clusters have a molar ratio (x) of ruthenium (Ru) to the chalcogen (S, Se) in the range between 0.5 and 2. Such clusters have a special resistance to electrolyte acids and, due to the 8-ring structure in the case of sulfur and red selenium, produce properties which are particularly favorable for the synthesis.

According to a further embodiment of the invention, it is particularly favorable if the clusters with ruthenium (Ru) as transition metal and selenium (Se) as chalcogen are in the stoichiometric form (Ru) _n Se, where (n) is between 1, 5 and 2, in particular at 1, 7 lies. Clusters with this stoichiometric structure as a catalyst material meet the demands for high catalytic and selective activity to a particularly excellent degree.

The cathode according to the invention is effectively protected against chemical attacks by the thin surface coating made of the catalytically and selectively highly reactive material, so that no expensive cathode structures with platinum or similar noble metals or compounds are required. Therefore, according to a continuation of the invention, it is possible that the cathode has a porous and electrically conductive support. The porosity results in a high specific Cathode surface for the reactions. The carriers can be inexpensive substrates, in particular made of carbon black or norit-type carbon, which are particularly suitable for use in a gas diffusion electrode in fuel cells. Gas diffusion electrodes are constructed in three phases with a hydrophobic and a hydrophilic layer and an intermediate reactive solid layer. The use of conventional substrates, such as, for example, glazed carbon or indium tin oxide (ITO) substrates, is also possible.

In fuel cells with an alkaline electrolyte, contamination of the electrolyte by carbon dioxide can occur, which causes carbonate to form in alkaline solutions. This reduces the conductivity of the electrolyte and the life of the electrodes. It is therefore cheaper to design fuel cells with an acidic medium, in particular according to an embodiment of the invention with sulfuric acid as the liquid electrolyte, which is used most frequently. The use of phosphoric acid is also possible, but this is only conductive at high temperatures (300 ° C).

A direct methanol fuel cell works at atmospheric or overpressure and working temperatures between 80 ° C and 100 ° C. The high-energy methanol is cracked for hydrogen production without redirection via a reformer. Therefore, in a further embodiment of the invention, it is advantageous if the methanol fuel cell is designed as a direct fuel cell.

The process for producing the above-described inert cathode for selective oxygen reduction is characterized according to the invention in that, in order to produce thin surface coatings in the nm range, the porous support of the cathode is immersed in the catalyst material in the form of a colloid solution, then dried at room temperature and then at a temperature between 200 ° C and 300 ° C, in particular at 208 ° C, is annealed. Such a process can be carried out without great expenditure in terms of equipment and costs and is achieved with a one-off Run best results. By immersing it in the colloidal catalyst material, the cathode surface is evenly coated with a liquid surface. After the coating has dried, in particular in order to accelerate the drying process, even in vacuo, the solid constituents of the colloid are fixed on the carrier in a colloidally disperse, homogeneous distribution. Foreign substances that are still present can then simply be evaporated by the tempering process without damaging the catalyst material.

According to a further embodiment of the invention, the colloid solution is advantageously designed such that the clusters float in an organic solvent with the appropriate addition of a long-chain stabilizer in a colloidally disperse homogeneous distribution, the boiling point (Tss) of the stabilizer being above the boiling point (T _SL ) of the solvent . With the help of such a colloid solution, the required cluster material can be made available in a tailored manner. The solvent is the carrier. The long-chain stabilizer prevents the clusters from sticking together to form a powder. The amount to be used depends on the cluster occurrence in the solvent and can be determined via the molar ratio of the individual components. The reaction takes place at the boiling point (T _S L) of the solvent almost independent of the components used. The position of the boiling point (Tss) of the stabilizer above the boiling point (T _S L) of the solvent ensures that the solvent can be removed without affecting the stabilizer and thus the fine distribution of the clusters. The stabilizer is only removed in the annealing process, so that only the colloidally dispersed, homogeneously distributed clusters are then on the cathode surface and form the solid coating.

The preparation of the colloidal solution can be carried out of the solvent by a wet-chemical-organic synthesis in continuation of the invention is particularly simple and inexpensive in the region of the boiling temperature (T _S). This manufacturing process takes place in analogy to the manufacturing process of known catalyst materials in powder or thin film form. It is based on the combination of one or more metal carbonyls with a chalcogen in the solvent. In the invention there is also the stabilizer.

The range of metal carbonyls M (also as a combination of several metal carbonyls) is extensive and contains in particular:

• M (CO) ₆ with M = Mo, W

• M ₃ (CO) ι ₂ with M = Ru, Os • M ₄ (CO) ι ₂ with M = Co, Rh, Ir

• M ₆ (CO) i6 with M = Co, Rh, Ir

The range of chalcogens includes X = Se, Te, S. Here the 8-ring chalcogens, e.g. red selenium and sulfur, due to their purity particularly good properties for synthesis.

Suitable organic solvents are:

Toluene C ₆ H ₅ CH ₃ T _SL = 111 ⁰ C

• xylene C ₆ H (CH ₃ ) ₂ T _S = 139 ° C / 140 ° C • mesitylene C ₆ H3 (CH ₃ ) 3 T _SL = 165 ° C

The range of long chain stabilizers is also extensive. The following are particularly suitable:

• Thiols eg 1-nonanthiol C ₉ H ₂₀ ST _ss = 220 ° C 1-dodecanethiol Cι ₂ H ₂₆ ST _ss = 226 ° C

1-octadecanethiol Cι ₈ H ₃₈ ST _S s = 185 ° C

• Phenols, for example 2,4-di-tert-butylphenol [(CH3) ₃ C] ₂ C ₆ H ₃ OH T _ss = 265 ° C

2,6-di-sec-butylphenol [C ₂ H5 (CH ₃ )] 2C6H ₃ OH; Tss = 255 ° C 2,6-di-tert-butylphenol [(CH ₃ ) ₃ C] ₂ C ₆ H ₃ OH T _ss = 253 ° C

According to the invention, it is particularly advantageous if the solvent is xylene with a boiling temperature (T _S ι_) of 140 ° C and the stabilizer is 1-octadecanethiol with a boiling temperature (T _S s) of 185 ° C. In connection with the further Embodiments of the inert cathode according to the invention explained above, in which the clusters with ruthenium (Ru) as transition metal and selenium (Se) as chalcogen are in the stoichiometric form (Ru) _n Se, where (n) between 1, 5 and 2, in particular in 1, 7, and a tempering temperature of 208 ° C all components are given in order to obtain an optimal cathode with excellent catalytic and selective properties. Only about 3.1 ng of catalyst material per cm ^{2 of} active electrode area are required due to its colloidal dispersion, which is extremely inexpensive.

The synthesis of the catalyst material is briefly explained below:

In a flask with a reflux condenser, the solvent xylene (100 ml) is purged with argon for 10 minutes to remove oxygen that would react with the components. Then powdered selenium (18 mg; 22.8 μM) is dissolved in the xylene by heating to 140 ° C. and the solution is then cooled again to room temperature. The semiconducting transition metal carbonyl triruthenium dodeca-carbonyl (Ru ₃ (CO) ι ₂ ; 72.9 mg; 11.4 μM) and the stabilizer 1-octadecanethiol (220 mg; 76.9 μM) are added. The amount of stabilizer is approximately three times the amount of carbonyl at a molar ratio of 6.75. The cluster formation then takes place after heating to the xylene boiling point (T _S L) of 140ΦC over a period of approximately 20 h. During the reaction, the colloid is constantly stirred and cooled by reflux.

The addition of a substrate for depositing a powder or a thin layer during cluster formation, as is known from the prior art, is not necessary in the production of the colloid. The yield is 100%, ie all material brought in is also converted. There are no losses, for example due to deposits on the inside of the piston, as are known in the case of powder formation. The clusters formed have a size of 3 nm to 4 nm. The stoichiometric composition according to the Rutherford backscattering technique (RBS), measured on a thin layer of 6 nm to 7 nm on a glassy carbon substrate (Glassy Carbon GC), results in Ru _{1] 7} Se, as described in a particularly preferred embodiment of the invention.

The figure shows the negative profile of the electrochemical oxygen reduction currents depending on the electrode potential (NHE - normal hydrogen electrode, reference electrode) for several identical, tempered glass electrodes made of glazed carbon, each with a thin layer of 5 μl of colloidal solution of the catalytically active material Ruι _ι7 They are covered, shown in 0.5 M sulfuric acid. The vertical lines along the curve represent the small error bars for the multiple measurements. The course remains unchanged after the addition of 1 M methanol, which indicates the high selectivity of the inert cathode according to the invention. The activation range lies in the lower straight line range of the curve and is better than that of platinum when using methanol, which proves the good catalytic property of the cathode according to the invention with its homogeneous surface coating, which was derived from a colloidally disperse colloid.

Claims

claims 1. Inert cathode for selective oxygen reduction in acidic medium by multi-electron transfer for electrochemical energy conversion in a methanol fuel cell with a thin surface coating of a selectively active catalyst material, based on semiconducting clusters of at least one transition metal and a chalcogen, characterized in that the thin surface coating has a degree of coverage generated from a small amount of material in the ng range per cm ² effective cathode area from individual clusters in a koiloid-disperse homogeneous distribution. 2. Inert cathode according to claim 1, characterized in that the transition metal is ruthenium (Ru) and the chalcogen is sulfur (S) or selenium (Se) and the cluster is a molar ratio (x) of ruthenium (Ru) to the chalcogen (S, Se) in the range between 0.5 and 2. 3. Inert cathode according to claim 1 or 2, characterized in that the clusters with ruthenium (Ru) as transition metal and selenium (Se) as chalcogen are in the stoichiometric form (Ru) _n Se, where (n) between 1, 5 and 2, in particular 1, 7. 4. Inert cathode according to one of claims 1 to 3, characterized in that the cathode has a porous and electrically conductive carrier. 5. Inert cathode according to one of claims 1 to 4, characterized in that the acidic medium is generated by sulfuric acid as a liquid electrolyte. 6. Inert cathode according to one of claims 1 to 5, characterized in that the methanol fuel cell is designed as a direct fuel cell. 7. A process for producing an inert cathode for selective oxygen reduction in an acidic medium by multi-electron transfer for electrochemical energy conversion in a methanol fuel cell with a thin surface coating made of a selectively active catalyst material, based on semiconducting clusters of at least one transition metal and a chalcogen, characterized in that that to produce thin surface coatings in the nm range, the porous support of the cathode is immersed in the catalyst material in the form of a colloid solution, then dried at room temperature and then tempered at a temperature between 200 ° C and 300 ° C, especially at 208 ° C becomes. 8. The method according to claim 7, characterized in that the colloid solution is designed such that the clusters float in an organic solvent with appropriate addition of a long-chain stabilizer in a koiloid disperse, homogeneous distribution, the boiling temperature (T _S s) of the stabilizer above Boiling point (TSL) of the solvent. 9. The method according to claim 8, characterized in that the colloid solution in the boiling point (TSL) of the solvent can be synthesized wet-chemically and organically. 10. The method according to any one of claims 7 to 9, characterized in that the solvent is xylene with a boiling temperature (TSL) of 140 ° C and the stabilizer 1 -octadecanethiol with a boiling temperature (Tss) of 185 ° C.