DE10225571A1 - Catalyst production, e.g. for fuel cell, comprises co-deposition of catalytically active metals on to substrate electrode inn electrolyte bath with pulsed EMF applied across electrodes - Google Patents

Abstract

The production of a catalyst comprises the co-deposition of two or more catalytically active metals on to a substrate immersed in a bath of an aqueous electrolyte, whilst a controlled pattern of EMF pulses is applied between the substrate and a counter electrode. The amplitude of the initial pulse is greater than that of succeeding pulses. An independent claim is included for a catalyst produced as above.

Description

The invention relates to a method for producing a Catalyst by electrochemical co-deposition of Metals, a catalyst that is obtainable by this process is as well as the use of this catalyst in electrochemical cells.

Electrochemical process for the preparation of catalysts are known from the prior art, for. B. from the Documents WO 00/56452, EP 106 197 A2 and US 6,080,504.

In the document WO 00/56452 (DaimlerChrysler) is a Process for the electrochemical preparation of a catalyst disclosed in which the catalytically active material on a Metal substrate is deposited by the to be coated Substrate with current or voltage pulses predetermined Amplitude and / or frequency is applied.

In the document EP 106 197 A2 (International Business Machines Corporation) is an electrochemical process for Formation of catalyst crystallites on a substrate discloses that a cathode is first formed with a nucleation Pulse is charged with high voltage and then with a longer crystal growth pulse with lower voltage.

The document US Pat. No. 6,080,504 describes a method for Deposition of nanocrystalline particles of a catalytically active Metal disclosed on a substrate. In the process, the substrate becomes with sequences of current pulses in certain temporal Distances (frequencies) applied.

The electrochemical co-deposition of catalytically active Metals on substrates such. B. graphite paper or Carbonaceous materials often cause difficulties. For example, the deposited metals often adhere poorly or there are competition reactions such. B. Hydrogen evolution, the disadvantage of the deposition process influence.

On the other hand, however, are just catalysts that two or more catalytically active metals are widely used by great interest. In the field of fuel cell Technology are, for example, catalysts which the Metals include platinum and ruthenium due to their high levels Tolerance to CO poisoning of particular interest.

Unfortunately, the electrochemical process is currently being prepared Co-deposition of these two metals (Pt and Ru) as a result of strong different reduction potentials special Trouble. The reason for this is likely to be particularly in the complex Deposition behavior of the metal ruthenium be sought. For example, its exact deposition potential have been unknown until recently (see, for example, A.J. Bard (Ed.), Encyclopaedia of electrochemistry of the elements, Vol. 6, Dekker-Verlag 1976). Also the electrochemical Co-deposition of other metals prepares z. T. considerable Trouble.

An object of the present invention is therefore, a improved process for the preparation of a catalyst in which a substrate is in a one-pot reaction with at least two metals is coated.

Other objects of the present invention are a To provide catalyst of at least two metals and to show a use for this catalyst.

Surprisingly, it has been found that by means of a electrochemical deposition process from a metal salt containing solution in which a substrate with a sequence voltage pulses are applied, well-adhering, CO tolerant catalysts are available, even at relatively high CO concentrations are not complete be poisoned.

A first object of the present invention is therefore a method for producing a catalyst by electrochemical co-deposition of two or more catalytically active metals on a substrate, where the substrate in an electrolyte is dipped, which is the Contains catalyst metals, and at least temporarily an electrical Voltage between the substrate and a counter electrode is applied, the catalyst metals directly with the Substrate firmly connected. In this procedure will be According to the invention the catalytically active metals on a Substrate deposited by the substrate to be coated is applied with a sequence of voltage pulses.

The two or more catalytically active metals are used in the hereinafter abbreviated to the term catalyst metals.

The catalyst metals are preferably Platinum metals, d. H. metals from the group Ru, Os, Rh, Ir, Pd, Pt, more preferably around the metals Pt and Ru and in order Mixtures of these metals.

The electrolyte contains the catalyst metals in at least partially, preferably completely dissolved form. In particular, water is suitable as the solvent. To produce a suitable electrolyte, the catalyst metals are preferably dissolved in the form of their metal salts, which are at least partially soluble in the particular solvent, preferably completely soluble. Preferably suitable metal salts are, for. B. water-soluble salts of Pt and Ru, z. B. H ₂ [PtCl ₆ ] .6H ₂ O and RuCl ₃ .xH ₂ O. But also water-soluble salts of other catalyst metals are suitable, for. B. RhCl ₃ .3H ₂ O, Pd [OOCCH ₃ ] ₂ , OsCl ₃ .xH ₂ O, IrCl ₄ .xH ₂ O, FeCl ₃ .6H ₂ O, CoCl ₂ .6H ₂ O, NiCl ₂ .6H ₂ O etc.

Preferred electrolytes contain ions of at least two different catalyst metals M ₁ and M ₂ , in particular Pt and Ru ions, in the ratio (in% by weight) of M ₁ : M ₂ -10: 1 to 1: 5, more preferably in the ratio 3: 1 to 1: 3. By adjusting this ratio, the properties of the catalyst can be influenced. A higher proportion of M ₁ , especially Pt, gives a catalyst with a higher activity; a higher proportion of M ₂ , in particular Ru, gives a more cost-effective catalyst. In general, however, the highest possible activity is sought at the lowest possible cost.

In the context of the present invention suitable substrates z. B. gas diffusion electrodes (in short: GDL). The GDL exist usually made of coarse-pored carbon fiber paper, e.g. B. Graphite paper, or carbon fiber fabric with porosities up to 90%. Around the flooding of the pore system with that at the cathode to prevent the formation of reaction water, these materials impregnated with hydrophobic materials, e.g. B. with Polytetrafluoroethylene (PTFE). At the impregnation can be a Calcination at about 340 to 370 ° C to connect to the Melt PTFE material. To improve the electrical contact between the catalyst layers and the Gas diffusion layers are commonly used on the respective catalyst layer facing side with a so so-called compensation layer of carbon black and a fluoropolymer Coated, the porous and water repellent and at the same time is electrically conductive and also a reasonably smooth Has surface.

The GDL can z. B. using a pasty Preparation by printing, knife coating, rolling or spraying on the Graphite paper are applied. The paste-shaped Preparations usually contain a soluble, proton conductive material, one or more solvents and, if necessary, highly dispersed, hydrophobic materials and pore formers.

As graphite paper is z. B. TGP 090 Toray suitable. As a carbonaceous component comes z. B. graphitized Soot, z. B. C50 Chevron, in question. As organic Polymer comes z. As polytetrafluoroethylene (PTFE) in question. In this regard are suitable for. For example, PTFE dispersion, z. B. Hostaflon TF5235 from Dyneon.

Under a sequence of voltage pulses is in the context of present invention, a series of voltage pulses Roger that. Synonymous with this, the term sequence of Voltage pulses are used. The number of voltage pulses depends essentially on the difference of the amplitudes of the individual voltage pulses. The smaller this difference is, the more voltage pulses are needed to the At least approximately completely precipitate catalyst metals. Out For reasons of efficiency is the lowest possible number Voltage pulses preferred (time saving). It has, however shown that a larger number of voltage pulses with different amplitude in terms of possible complete separation of the catalyst metals is advantageous. Therefore, the series of voltage pulses consists of at least three voltage pulses, but preferably at least five Voltage pulses and in particular at least seven Voltage pulses, but it can also, in special cases, off There are considerably more voltage pulses.

Under the amplitude of a voltage pulse, the maximum Value of the voltage understood during the duration of the Pulse occurs.

As voltage pulses are within the scope of the present Inventions preferably suitable pulses of DC voltages. Further Also suitable are DC pulses that with Alternating voltages are superimposed.

With regard to the pulse shape are in particular rectangular pulses suitable. Also suitable are triangular pulses, sine pulses, Sawtooth pulses, etc.

The inventive method allows the production a catalyst by depositing two or more Metals from a deposition bath or an electrolyte (One-pot reaction), in particular of platinum and ruthenium, and is therefore, compared to prior art methods associated with reduced effort.

Preferably, the sequence of voltage pulses begins with a first voltage pulse having the largest amplitude of all having applied voltage pulses.

By this first voltage pulse crystal nuclei are that Metal deposited on the substrate, which is the highest Deposition potential of all in the electrolyte has present metals (hereinafter briefly first metal called, for. Pt). In the context of the present invention was namely found that crystal nuclei of a metal with high Deposition potential, especially Pt, especially good on the Adhere surface of the substrate.

The first voltage pulse is then preferably from followed by further voltage pulses with lower amplitudes. In this case, more metal separates and is deposited on the crystal nuclei formed during the first voltage pulse so that there is a crystal growth there. Thereby particularly well-adhering catalyst particles develop on the Substrate surface, which also partially poorly adherent Contain metals with lower deposition potential can. The growing through the metal deposit If necessary, catalyst particles can form a closed layer unite. However, in the context of the present invention also an un-closed layer or insulated present catalyst particles as a catalyst layer designated.

It is preferred if the amplitudes of the on the first voltage pulse following further voltage pulses gradually increase.

In this case, the amplitudes of the voltage pulses are preferably according to the deposition potentials or reduction potentials predetermined the deposited catalyst metals. Ie. in particular, that the amplitude of the first voltage pulse in essentially the deposition potential of the first metal corresponds to the amplitude of the second voltage pulse in essentially the deposition potential of the second metal and that the amplitudes of the further voltage pulses between these limits.

• a) In einem ersten Schritt beaufschlagt man das Substrat mit einem ersten Spannungspuls, dessen Amplitude gleich hoch ist oder unwesentlich höher als das höchste Abscheidungspotential der abzuscheidenden Katalysatormetalle, um Keime dieses Katalysatormetalls auf dem Substrat zu bilden.

It is further preferred if the amplitudes of the applied voltages are specified as follows:

• a) In a first step, applied to the substrate with a first voltage pulse whose amplitude is equal to or slightly higher than the highest deposition potential of the deposited catalyst metals to form nuclei of this catalyst metal on the substrate.

• a) In einem zweiten Schritt beaufschlagt man das Substrat mit einem zweiten Spannungspuls, dessen Amplitude gleich hoch ist oder unwesentlich höher als das niedrigste Abscheidungspotential der abzuscheidenden Katalysatormetalle, um dieses Katalysatormetall wenigstens teilweise abzuscheiden.

By insignificantly higher, it is understood that the amplitude of the first voltage pulse is at most 100 mV above the deposition potential of the first metal, preferably at most 50 mV and in particular at most 25 mV.

• a) In a second step, applied to the substrate with a second voltage pulse whose amplitude is equal to or slightly higher than the lowest deposition potential of the deposited catalyst metals to at least partially deposit this catalyst metal.

• a) In weiteren Schritten beaufschlagt man das Substrat gegebenenfalls mit einem oder mehreren weiteren Spannungspulsen, deren Amplituden jeweils höher sind als die Amplituden der jeweils vorangegangenen Spannungspulse, um die Katalysatormetalle wenigstens teilweise abzuscheiden.

Insignificantly higher is understood that the amplitude of the second voltage pulse is at most 100 mV above the deposition potential of the second metal, preferably at most 50 mV and in particular at most 25 mV.

• a) In further steps, the substrate is optionally applied to one or more further voltage pulses whose amplitudes are each higher than the amplitudes of the respective preceding voltage pulses in order to at least partially deposit the catalyst metals.

• a) in einem weiteren Schritt beaufschlagt man das Substrat mit einem weiteren Spannungspuls, dessen Amplitude höher ist als die Amplitude des vorangegangenen Spannungspulses und niedriger als die Amplitude des ersten Spannungspulses, um die Abscheidung der Katalysatormetalle wenigstens annähernd zu vervollständigen.

The amplitude in the further voltage pulses may differ by 10 to 250 mV, preferably by 50 to 200 mV and in particular by 75 to 125 mV.

• a) in a further step applied to the substrate with a further voltage pulse whose amplitude is higher than the amplitude of the previous voltage pulse and lower than the amplitude of the first voltage pulse to at least approximately complete the deposition of the catalyst metals.

The amplitudes of the second and subsequent ones Voltage pulses are preferably smaller or at most equal like the highest amplitude, d. H. the amplitude of the first Voltage pulse.

As mentioned, well-adhering with the first voltage pulse Crystal nuclei formed on the substrate surface. This Crystal seeds essentially consist of the first metal (eg Pt). With a second voltage pulse, the Having the smallest amplitude, then can further metal, which in the essentially consists of the second metal and that in the general a lower adhesion on the Substrate surface (eg, Ru), are deposited.

Regarding the deposition potentials is at this point referred to the electrochemical series, in the the standard electrode potentials corresponding Redox pairs are listed in ascending order. On Standard electrode potential is to a certain extent the deposition potential of a Redox pair under standard conditions (reference electrode = Normal hydrogen electrode, activity = 1, temperature = 298.15 K, Pressure = 101.325 kPa), d. H. that this Standard electrode potentials of corresponding redox pairs as a measure of their Abscheidungspotential can be taken. The Deposition potentials depend on u. a. from the substrate, the Electrolytes, in particular its concentration, and the properties of the respective catalyst metal.

A low deposition potential therefore have z. For example, such ions that are high in the voltage series, such. As the redox couple Li / Li ⁺ , which has a standard electrode potential of -3.045 V. On the other hand, a high deposition potential has such ions which are far below in the voltage series, such as, for example, B. the redox couple Au / Au ⁺ , which has a standard electrode potential of +1.42 V.

In general, with just one more voltage pulse not the complete amount of that present in the electrolyte second metal (eg Ru) are deposited. In practice it is observed that the measurable current is due to Metal deposition during the second voltage pulse after a certain Time goes back to zero, though still depositable quantities of the second metal (eg, Ru) are present in the electrolyte.

If the substrate subsequently with another Voltage pulse applied, which has a higher amplitude as the preceding voltage pulse, so it is surprising again a current measurable, which is its cause in further itself has depositing metal. Ie. it separates another metal consisting essentially of the second metal (eg Ru) consists.

By further voltage pulses, the deposition of the second At least approximately completed metal. That I Although the metal that separates it initially exists in the essentially from the second metal (eg Ru) the proportion of metal but with higher deposition potential (eg Pt) decreases with increasing amplitude of the corresponding voltage pulse steadily increasing, which also causes the deposition of the first metal is at least approximately completed.

A second object of the present invention is a Catalyst which, according to the above disclosed process according to the invention is available.

The catalyst according to the invention adheres well to the substrate and is CO tolerant. Thus, the catalyst according to the invention not even at relatively high CO concentrations completely poisoned and regenerates upon access of Oxygen easily.

A third object of the present invention is the Use of a catalyst according to the above disclosed method according to the invention is available, in electrochemical cells, devices for generating hydrogen-containing gases, so-called reformers, and devices for cleaning hydrogen-containing gases.

The o. G. electrochemical cells or devices can, if they contain the catalyst according to the invention, with Be operated fuels that have a relatively high content to CO, without fully their performance to lose. As a result, z. B. the effort in cleaning from Z. B. reformate gases can be reduced.

Preferably, the use of the invention Catalyst in fuel cells in question, in particular in PEM Fuel cells.

Further preferred is the use of the invention Catalyst as electrode coating.

The invention will be explained in more detail with reference to FIGS. 1 to 4 explained below.

Showing:

Fig. 1 a series of eight rectangular voltage pulses according to the invention;

FIG. 2 shows a likewise sequence according to the invention consisting of seven rectangular voltage pulses; FIG.

Fig. 3, the U / CO characteristic of an electrode made by the inventive process;

Fig. 4, the U / air bleed characteristic of an electrode produced by the process according to the invention.

In Fig. 1, a sequence of voltage pulses according to the invention is shown schematically, with which a substrate for the electrochemical co-deposition of catalyst metals is applied. The sequence begins with a first voltage pulse ( 1 ), which is a high-voltage pulse and whose amplitude corresponds to the deposition potential U _A (M ₁ ) of the metal M ₁ ("first metal", eg Pt).

With this first voltage pulse ( 1 ) form crystal nuclei of the metal M ₁ , which has the highest deposition potential. From a Pt and Ru ions containing electrolyte such. B., with a corresponding high voltage pulse, crystal nuclei are formed by Pt, but not with a corresponding low-voltage pulse crystal nuclei of Ru.

The first voltage pulse ( 1 ) is directly followed by a second voltage pulse ( 2 ) whose amplitude corresponds to the deposition potential U _A (M ₂ ) of the metal M ₂ ("second metal", eg Ru). As can be seen from FIG. 1, U _A (M ₂ ) is smaller than U _A (M ₁ ).

The second voltage pulse ( 2 ) is followed by the further voltage pulses ( 3 ), ( 4 ), ( 5 ), ( 6 ), ( 7 ) and ( 8 ). Overall, therefore, in this sequence, eight voltage pulses are applied. Of course, it is also possible to apply more or less than eight voltage pulses. With more voltage pulses, metal deposition can be made more complete; with less voltage pulses, the process becomes more efficient because it is shorter in time. All subsequent voltage pulses ( 2 ) to ( 8 ) have an amplitude which increases with their number. While in this example the increase of the amplitudes is relatively uniform, it is also possible that the increase from voltage pulse to voltage pulse is larger or smaller. In Fig. 1, the pulse durations P _n (n = 1-8) steadily increase. However, the pulse durations P _n can also be the same or increase or decrease from voltage pulse to voltage pulse. By suitable choice of the pulse durations of the applied voltage pulses, the amount of catalyst, the position of the nuclei, the crystal growth and the distribution of the co-catalyst can be adjusted.

The sequence ends with the last voltage pulse ( 8 ), which has approximately the amplitude of the first voltage pulse, but not reached. With this last voltage pulse preferably as much M ₁ , in particular Pt, deposited in order to obtain a catalyst as active as possible.

In Fig. 2 shows another preferred variant of the inventive method is shown schematically. Fig. 2 corresponds essentially to Fig. 1, but especially with the difference that the voltage pulses do not follow each other directly, but that between the voltage pulses waiting times W _n (n = 1-6) are provided. Furthermore, the sequence in FIG. 2 has only 7 voltage pulses ( 1 ) to ( 7 ). Waiting times between the voltage pulses can be particularly advantageous if the deposition bath between the voltage pulses sufficient time for mixing or homogenization, for. B. by stirring.

The invention will be explained in more detail with reference to an embodiment and FIGS. 3 and 4.

example

It was a catalyst according to the invention by electrochemical co-deposition by the process according to the invention produced. The electrode thus obtained was subsequently examined for their CO tolerance.

1. Preparation of a catalyst according to the invention

A deposition bath containing an aqueous electrolyte having the following compositions was used:
Sulfuric acid 0.1 mol / L
H ₂ [PtCl ₆ ] .6H ₂ O 5 g / L (9.65 mmol / L)
RuCl ₃ .xH ₂ O 5 g / L (24.10 mmol / L)

All o. G. Chemicals were manufactured by Merck Eurolab GmbH based.

Further, a substrate was used, which was prepared as follows:
As a base, a graphite paper was used, to which 2 gas diffusion layers (GDL) were applied, namely a lower gas diffusion layer and an upper gas diffusion layer.

The bottom gas diffusion layer was applied at a coverage of 0.9 mg / cm ² by knife coating and contained conductive, graphitized carbon black and 11% PTFE.

The upper gas diffusion layer was applied at a coverage of 0.6 mg / cm ² by knife coating and contained conductive, graphitized carbon black and no PTFE.

As graphite paper TGP 090 Toray was used. As a conductive, graphitized carbon black C50 was the company Chevron used. As PTFE (polytetrafluoroethylene) became a PTFE dispersion from Dyneon (Hostaflon TF5235) used.

The thus prepared substrate was removed by means of a Clamping devices arranged in the electrolyte and as a cathode connected. The anode was a platinum-plated Titanium expanded metal mesh used.

The cathode was charged with the following eight voltage pulses (see Table 1): TABLE 1

The quantity ratio of the deposited metals on the Cathode was Pt: Ru = 1.5: 2.

2. Examination of the CO tolerance of the invention Catalyst without air bleed

The catalyst-coated electrode according to the method of the present invention was incorporated into a test PEM fuel cell, which was then operated with hydrogen gas at a current density of 1 A / cm ² . Five experiments were carried out in which the hydrogen gas was mixed with different amounts of CO, so that hydrogen gas with five different CO concentrations was obtained. The CO gas added with CO simulates the reformate gas produced in a reforming process.

The test PEM fuel cell was an FC50-OISP Fuel cell of the company QuinTech (www.quintech.de) used.

The measurement result is shown in Fig. 3, Table 2 summarizes the measurement data together. Table 2

Fig. 3 shows that the electrode of pure hydrogen as fuel (0 ppm CO) of course provides the highest voltage (measuring point 1 ). In the case of hydrogen with a CO concentration (impurity) of 50 ppm, the voltage falls to about half (measuring point 2 ), but then decreases only slightly at the following 3 measuring points ( 3 ) to ( 5 ), respectively It stays almost constant. In particular, it should be noted that the catalyst prepared by the process according to the invention does not completely poison the electrode used here, even at CO concentrations as high as 200 ppm, but allows a relatively high voltage of 287 mV.

3. Investigation of the CO tolerance of the invention Catalyst with air bleed

The test PEM fuel cell described under 2.) was now operated with hydrogen with a constant CO concentration of 50 ppm, again at a current density of 1 A / cm ² . In contrast to 2.), however, air was added to the hydrogen in increasing amounts (air bleed, in% air per volume H ₂ ).

The measurement result is shown in Fig. 4, Table 3 summarizes the measurement data together.

Fig. 4 shows that, as expected, the lowest voltage is measured at the measuring point ( 1 ). Furthermore, FIG. 4 shows that the catalyst produced by the process according to the invention regenerates rapidly even when small amounts of air are metered in. The catalyst is detoxified by oxidizing CO bound to the surface with air-O _{2 of} the air bleeds. Thus, when adding only 1% by volume of air to the hydrogen, a voltage of 398 mV is already measurable (measuring point 2 ) and at an addition of only 3% by volume of air, an almost equally high voltage can be measured (measuring point 4 ), as in operation with pure hydrogen ( Fig. 3, measuring point 1 ).

The example shows that with the method according to the invention in a simple way (one-pot procedure!) a relatively stable, CO-tolerant catalyst is available.

Claims (10)

A process for producing a catalyst by electrochemical co-deposition of two or more catalytically active metals on a substrate, wherein the substrate is immersed in an electrolyte containing the catalyst metals and at least temporarily applying an electrical voltage between the substrate and a counter electrode wherein the catalyst metals are fixedly connected directly to the substrate, characterized in that the catalytically active metals are deposited on a substrate by applying a sequence of voltage pulses to the substrate to be coated. 2. The method according to claim 1, characterized, that the sequence of voltage pulses with a first Voltage pulse begins, which has the largest amplitude. 3. The method according to claim 2, characterized, that the first voltage pulse of further voltage pulses is followed with lower amplitudes. 4. The method according to claim 3, characterized, that the amplitudes of the on the first voltage pulse following further voltage pulses gradually increase. 5. The method according to any one of claims 1 to 4, characterized, that the amplitudes of the applied voltages according to the deposition potentials of the deposited Catalyst metals are specified. 6. The method according to claim 5, characterized in that the amplitudes of the applied voltages are specified as follows: a) applying a first voltage pulse whose amplitude is equal to or slightly higher than the highest deposition potential of the catalyst metals to be deposited to form nuclei of this catalyst metal on the substrate; b) applying a second voltage pulse whose amplitude is equal to or slightly higher than the lowest deposition potential of the catalyst metals to be deposited in order to at least partially deposit this catalyst metal; c) optionally applying one or more further voltage pulses whose amplitudes are each higher than the amplitudes of the respective preceding voltage pulses in order to at least partially deposit the catalyst metals; d) applying a further voltage pulse whose amplitude is higher than the amplitude of the previous voltage pulse and lower than the amplitude of the first voltage pulse to at least approximately complete the deposition of the catalyst metals. 7. Catalyst obtainable by a process according to one of claims 1 to 6. 8. Use of a catalyst by a process can be obtained according to one of claims 1 to 6, in electrochemical cells and devices for generating or Purification of hydrogen-containing gases. 9. Use according to claim 8, characterized, that it is the electrochemical cells to Fuel cells is, preferably to PEM fuel cells. 10. Use according to claim 7 or 8, characterized, that the catalyst used as an electrode coating becomes.