DE2216192A1 - METAL PHOSPHIDE CATALYST FOR ELECTRODES OF ELECTROCHEMICAL CELLS - Google Patents

Description

SIEMENS AKTIENGESELLSCHAFT Erlangen, March 30, 1972 Berlin and Munich Werner-von-Siemens-3tr.50

Our reference: 2216192 ^VPA 72/7525 Bh / Koe

Metal phosphide catalyst for electrodes of electrochemical cells

The invention relates to a metal phosphide catalyst for electrodes, in particular anodes, of electrochemical cells, in particular fuel elements with acidic electrolytes, and a method for producing this catalyst.

It is already known, in electrodes for electrochemical cells, phosphides and disulfides of the IV. To VI. Subgroup of the periodic table of the elements belonging to metals molybdenum, tungsten, zirconium, mob, hafnium and tantalum use (U.S. Patent 3,573,988). The electrodes in which the particles of said materials are bound by a binder are held together, serve as cathodes in electrochemical cells with alkaline electrolytes.

From US Pat. No. 3,411,953 it is known to use a nickel-phosphorus alloy as a catalyst material for the anode in addition to nickel in fuel cells with alkaline electrolytes. The phosphorus content of the catalyst material is very low, it is in the range between 4 and 6 io. It is also known to use anodes which have nickel or cobalt phosphide as a catalyst in fuel cells, in particular with alkaline electrolytes, for the oxidation of hydrazine (US Pat. No. 3,451,852). The stoichiometric ratio between metal and phosphorus is 2: 1, ie Ni ₂ P or Co ₂ P is present.

It is known from US Pat. No. 3,449,169, in fuel cells To use platinum phosphide PtPp as an electrocatalyst. Electrodes with this catalyst can do both for anodic oxidation of fuels as well as for

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serve cathodic reduction of oxygen; the electrolyte used in the fuel cell can be acidic or be alkaline.

The catalysts mentioned are not yet satisfactory in various respects. The use of expensive metals, such as platinum in PtP ₂ , is contrary to the economic use of electrochemical cells, such as fuel elements. The natural occurrences of platinum are also limited. On the other hand, there is in particular a great need for catalysts that can be used in electrochemical cells with acidic electrolytes and, in turn, for catalysts for the anode. In addition, the catalysts should have a wide range of possible uses. Investigations have shown, for example, that the nickel or cobalt phosphide (Ni ₂ P and Co ₂ P) known for the anodic oxidation of hydrazine corrodes and is therefore unsuitable for the oxidation of hydrogen in fuel elements with acidic electrolytes.

The object of the invention is to provide a new metal phosphide catalyst for electrodes of electrochemical cells, through which the supply of effective electrocatalysts can be expanded considerably. The new metal phosphide catalyst aims in particular to increase the selection of active Enrich anode materials, with fuel elements with acidic electrolytes in Come into consideration. The new catalyst should also "be cheap and it should have a high catalytic efficiency Maintained over a long period of time, i.e. it should be stable and also insensitive to catalyst poisons.

This is achieved according to the invention in that the catalyst is at least one metal phosphide or one or more of the Metals of the first and seventh subgroups as well as the metals of the iron group of the periodic table of the elements with a

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Contains phosphorus content of the metal phosphide of at least 1 atom of phosphorus per metal atom. Among the metals of Iron group, which belong to the eighth subgroup of the periodic table of the elements, are the metals iron, cobalt and Understood nickel.

The metal phosphide catalyst according to the invention meets the requirements placed on electrocatalysts in general to an extraordinarily high degree and, in particular, it also has all the specially required properties. He shows one high activity, especially for anodic oxidation, and is extremely corrosion-resistant, especially in the acidic medium was both surprising and not must be designated predictably. The catalyst according to the invention is also able to maintain its activity for a long time and is relatively insensitive against catalyst poisons. Finally, the catalyst points still has the advantage of good electronic conductivity.

The catalyst according to the invention is particularly suitable for use in a fuel element with an anode containing this catalyst for the electrochemical conversion of hydrogen, hydrogen-containing compounds or organic fuels, a cathode for the electrochemical conversion of oxygen or oxygen-containing oxidizing agents, and an acidic electrolyte. When using hydrogen as fuel, the metal phosphide catalyst according to the invention is not poisoned by the carbon monoxide that is present in the hydrogen as a result of its production by reforming hydrocarbons or by decomposing methanol (CIUOH -. ^

2 Hp + OO) - may be included. As an organic fuel For example, formaldehyde can be used.

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The catalyst according to the invention can advantageously contain at least one of the following metal phosphides: copper phosphide, gold phosphide Au ₂ P, manganese phosphide MnP, iron phosphide or FeP, cobalt phosphide GoP or GoP and nickel phosphide NiP or NiPp. All of these phosphides have a relatively high phosphorus content; the phosphorus content in the nickel phosphides, for example, is about 50 i '(nip) or about 60 <fo (NiP). The metal phosphides from the above series show advantageous effects when used as anode materials in fuel cells with acidic electrolytes, and in particular in the electrochemical oxidation of hydrogen or hydrogen-containing gas mixtures. The phosphides mentioned show their catalytic activity even at low occupancy densities, for example about 100 mg / cm, and even at room temperature (about 22 ^° C.). CoP can be used particularly advantageously. With an electrode containing this electrocatalyst, for example, in the anodic oxidation of hydrogen at 70 C in 2.5 m HpSO., An occupancy of about 200 mg / cm and a polarization of 100 mV, measured against the reversible hydrogen potential, a Current density greater than 180 mA / cm. With CoP, recoverable high current density is also maintained in long-term experiments: A current density of 50 mA / cm (at 7O ⁰ C and 2.5 M H ₂ SO.), For example, can be maintained in the endurance test over a period of 1 000 hours. CoP can also be used advantageously for the anodic oxidation of formaldehyde. In addition, CoP is particularly insensitive to poisoning by CO. GoP, in particular, has the further advantage that it is a good electrical conductor. Powdery CoP shows good electrical conductivity even at room temperature, so that when it is used in electrodes there is no need for an additional electrically conductive carrier material; Such a carrier material is required, for example, for other electrode materials such as molybdenum disulfide (MoS ₂ ). In addition, CoP is cheap to manufacture and can be made in large quantities.

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The metal phosphide catalyst according to the invention can advantageously contain at least two different metals. this is to be understood that in a metal phosphide lattice the metal, for example copper or iron, partially through at least one other metal, such as cobalt, is replaced. It is therefore not a mixture of two or more Metal phosphides before, but a metal phosphide that has two or contains more metals. Such a catalyst has further improved properties, for example one increased activity, as the effectiveness of the superimposed on individual metals.

The metal phosphide catalyst of the invention can be advantageous be powdery, the powdery catalyst material then in an electrode containing this catalyst advantageously between a diaphragm, which is gas-tight in the electrolyte-soaked state, and a support structure, in particular a support net is built in. Sin such a lead electrode structure is known per se and is generally referred to as "supported electrode" (see for example: F.v.Sturm, "Electrochemical power generation", Verlag Chemie GmbH, Weinheim / Bergstr., 1969, page 109 ff.). With powdery Catalyst material, in particular, powders with grain diameters between 2 and 50 / u have proven to be suitable.

One containing a metal phosphide catalyst according to the invention Electrode can also advantageously be designed so that the powdery catalyst material with a Plastic binder is mixed and then the particles of the Catalyst material through the binder with each other and

if necessary, seal with an electrolyte-impregnated state Diaphragm are connected. The plastic binder must be resistant to the respective electrolyte liquid. When using sulfuric acid ^ as electrolyte liquid has a butadiene-styrene-Aerylnittfil-Copolynierisat as proved particularly advantageous. When used, hydrophobic

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Binder, the electrode can be made hydrophobic, which is particularly advantageous when the electrode is to be operated at low pressure.

In addition to the metal phosphide, the catalyst according to the invention can advantageously contain further catalytically active substances contain. This is particularly advantageous when electrodes containing such electrocatalysts are used for special Purposes. Organic semiconductors in particular have proven to be additional catalytically active substances, Tungsten carbide, for example in the form of silver-containing tungsten carbide, and the platinum metals have proven. Under the Platinum metals are understood to be the elements ruthenium, rhodium, palladium, osmium, iridium and platinum. As organic For example, semiconductors are particularly well suited to polymeric metal-containing and metal-free phthalocyanines (cf. US patent specification 3,585,079).

Finally, the metal phosphide catalyst can after Invention also additionally contain an electrically conductive carrier material. The carrier material makes the conductivity of the catalyst is improved and the resistance to current draw is reduced. Can be used as carrier materials Carbides, nitrides, borides and silicides of metals from IV. To VI. Subgroup of the periodic table of the elements and gold, Silver and carbon are used.

To produce the metal phosphides, for example, powdery phosphorus and powdery metal can be introduced into an ampoule in the appropriate stoichiometric ratio and heated ^{to a temperature of a few 100 ° C. for a longer period of time.} In the production of the metal phosphides, care is advantageously taken that an end product with a large surface area is obtained. Therefore, the production of the metal phosphide catalyst takes place in particular in ier way that a powdery metal or a metal compound

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bond at elevated temperature in the presence of an additive remaining in the end product, which is responsible for the formation of a large surface area, is reacted with phosphorus. As an additive that is insoluble and therefore im If the end product remains, activated carbon is preferably used.

The additive protects the metal powder or the powdery Metal compound during phosphidation before sintering together, whereby catalysts with a large active area are obtained. In addition, the addition of such Substances shorten the reaction time »The additives are compared to the starting materials and the reaction products inert and they have a melting point which is above the reaction temperature bsi the phosphidation.

In addition to such an additive, which is in the final product remains, can in the preparation of the Ifetallpliospliid catalyst Another additive can be used that is dissolved out of this after the completion of the catalyst. Sicii is particularly suitable for this purpose. the Use of such soluble excipients is at the same time submitted German patent application "Process for the production of particularly finely divided, homogeneous transition metal phosphides and chalcogenides ", Akt.Z. .......... (YPA 72/7526). Furthermore, there is also the possibility according to this suggestion only work with an additive au that is dissolved out of the finished catalyst.

It is particularly advantageous in the production of the metal phosphide catalyst the metal used in the form of a product obtained by reduction of metal salts, in particular by reduction of metal oxalates or metal carbonates will. Hydrogen in particular can serve as the reducing agent. In addition, the metal phosphide are advantageously prepared directly by reducing suitable compounds, for example by reducing metal

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pyrophosphates with hydrogen. Furthermore, metal phosphides produced in this way can also be in the air easy to handle, metal phosphides with a higher phosphorus content can advantageously be produced by phosphidation, for example CoP, from CoP.

The metal pyrophosphates are preferably produced by a precipitation reaction. It has been found to be beneficial it has been proven that the solution from which the metal pyrophosphate is precipitated contains an insoluble additive, like activated charcoal. Then, during the crystallization, carbon is converted into the individual crystallites of the Metal pyrophosphate is embedded and also penetrates its surface. During the subsequent reduction, it arises a microporous material that has a large active surface area. In the subsequent further reaction with Phosphorus - for the production of a metal phosphide with a higher phosphorus content - is sintered by the carbon and thus prevents the inner area from decreasing and the fine pores from disappearing. Determination of the surface the metal phosphides are expediently carried out according to the BET method, i.e. according to the method of Brunauer, Emmett and Teller (see, for example: S.J.Gregg, K.S.W. King, "Adsorption, Surface Area and Porosity", Academic Press, London and New York, 1967, p. 35 ff.).

The invention will be explained in more detail with the aid of a few figures and exemplary embodiments.

It shows

1 shows two characteristic curves of a metal phosphide catalyst according to the invention containing electrode, Fig. 2 and Fig. 3 further characteristics of electrodes with the metal phosphide catalyst according to the invention and FIG. 4 schematically a fuel element with one according to the invention Electrode containing catalyst.

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First of all, a preferred method of production of the catalyst according to the invention will be explained in more detail based on the production of a CoP ,, containing electrocatalyst.

example 1

To a solution of 145.5 g cobalt nitrate Co (NO.,) ₂ · 6 H ₉ O (0.5 mol) -in 0.5 1 of water are added at a temperature of about 2 ⁰ C a solution of 111.5 g lithium pyrophosphate · Na ₄ P ₂ O ₇ · 10 H ₂ O (0.25 mol) in 1.5 liters of water. In the process, cobalt pyrophosphate Co ₂ P ₂ O ₇ ° aq. Containing water of crystallization precipitates out. The precipitate is washed with water until the wash water no longer contains any nitrate, and then slurried in 300 cm of ethanol in order to completely remove the adhering water. It is then filtered off and the cobalt pyrophosphate obtained, which contains water of crystallization, is dried in air at room temperature. The yield of COgPpCU · aq. Is 93.2 g.

30 g of the powdery cobalt pyrophosphate obtained in this way are reduced in a hydrogen stream (100 l / h) at about 600 g in a tube furnace. The P _L eduktioii lasts about 4 hours, then it is cooled. In this way 14.5 g of powdered cobalt phosphide CoP are obtained.

5.2 g of this cobalt phosphide are mixed with 4 g of sodium fluoride NaF and then with 4 g of red phosphorus and melted in a thick-walled quartz ampoule under vacuum. The vial is heated for 14 hours at a temperature of about 65O ⁰ C. After the reaction has ended, the mixture is slowly cooled. During cooling, a part of the ampoule that does not contain any substance is always kept at a slightly lower temperature so that the excess phosphorus can collect there. After cooling, the reaction product is converted into the sodium fluoride with water, optionally under

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slight warming, removed. Using Debye-Scherrer recordings the end product obtained was examined and it was found that cobalt phosphide CoP is present.

The powdery GoP produced according to the described method is built into a supported electrode. For this purpose, the catalyst material is in 2.5 m H ₂ SO. suspended and sedimented on asbestos paper. A carbon fabric is then applied to the catalyst layer for support and contact. The catalyst coverage in the present case is

match about 200 mg / cm. The CoP used had a BET

p J

Surface of 3.1 m / g, the grain size of the CoP powder was about 10 to 20 / rev.

The characteristic of such an electrode is measured in a half-cell arrangement with an Hg / HgpSO ^ electrode as a reference electrode. In 2.5 m HpSO . as the electrolyte liquid, a current density of 52 mA / cm is obtained at a temperature of 70 ^{° C. and a polarization of 100 mV.}

The characteristic curve of this electrode shows curve 10 in Pig. 1. The current density i in mA / cm is plotted on the ordinate and the potential e of the electrode in mV, based on the hydrogen electrode as the reference electrode, on the abscissa. It should be noted that the characteristic curve shown was obtained at a hydrogen pressure of only 2 N / cm. The curve 11 in FIG. 1 shows the corresponding characteristic curve at an electrolyte ^{temperature of 22 °} C. and an operating pressure of 10 N / cm. The current density with a polarization of 100 mV is 10 mA / cm here.

Example 2

In addition to the method described in Example 1, in the aqueous solution of cobalt nitrate - before the addition of the Sodium pyrophosphate solution - 29.5 g of activated carbon while stirring

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brought in. When adding the sodium pyrophosphate, it beats the cobalt pyrophosphate then settles on the activated carbon. After the work-up described in Example 1, one obtains 134.4 g of a product consisting of activated carbon and cobalt pyrophosphate containing water of crystallization. It contains this product has equal amounts of cobalt and activated carbon by weight.

This product is made according to the procedure described in Example 1 - reduction, addition of ITaF and then Phosphidation - processed further to obtain an electrocatalyst consisting of CoP and activated carbon. Of the CoP-z ~ G content of the catalyst is about $ 72

A supported electrode prepared according to Example 1 with this CoP ^ / C catalyst are at 7O ⁰ C in 2.5 M H ₂ SO. with a catalyst coverage of 200 mg / cm and a polarization of 100 mV, a current density of 104 mA / cm.

The characteristic of such an electrode is shown by curve 20 in FIG. 2, in which the current density i is again plotted on the ordinate and the potential ε is plotted on the abscissa '. The characteristic curve shown in curve 20 was recorded at a hydrogen pressure of -17.5 ΪΤ / cm. A characteristic curve recorded with this electrode at an electrolyte ^{temperature of 22 °} C. - under otherwise identical conditions - shows curve 21 in FIG. 2. The current density with a polarization of 100 mV is 29 mA / cm.

Example 3

A metal phosphide catalyst is prepared according to the procedure described in Example 2, with the exception that no soluble additive, i.e. no sodium fluoride, is used. The metal phosphide catalyst consists of CoP, and coal with a content of about 72 wt. Yo CoP ,,.

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According to the manner described in Example 1, a supported electrode (loading density 200 mg / cm) is produced from this ΟοΡ, / C catalyst and measured in a half-cell. With a polarization of 100 mV, H ₂ SO is obtained in 2.5 m. at 70 ° 0 a current density of 185 mA / cm.

The characteristic of this electrode is shown in FIG. 3, in which the current density i is again plotted on the ordinate and the potential 6 (versus a reversible hydrogen reference electrode) is plotted on the abscissa. The hydrogen pressure was 10 N / cm ² .

Further improved results with regard to the current density can then be expected if one considers the production of the Catalyst pretends a smaller amount of coal and adds a little sodium fluoride. Due to the specific manufacture the carbon remains contained in the particles of the catalyst material, while the NaP plays the role of Carbon, which is found on the surface of the catalyst material. The NaP initially prevents the individual particles of the catalyst material from sintering together and reveals additional surface after being removed.

Pig. 4 shows schematically a fuel element with one containing the metal phosphide catalyst according to the invention Electrode. The fuel element consists of a housing with an electrolyte space 31 and two gas spaces 32 and 33. An anode 34 is arranged between the electrolyte space 31 and the one gas space 32, which the metal phosphide catalyst, for example GoP ,, contains. The powdery catalyst layer is supported by a carbon fabric 35 serving as a support structure against an asbestos cover layer 36 serving as a diaphragm pressed, which is gas-tight in the electrolyte-soaked state. The carbon fabric 35 serves at the same time for contacting and Power consumption. The electrolyte space 31 has a support structure 37, for example in the form of a polypropylene mesh. Between

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a cathode 38 is arranged in the other gas space 33 and the electrolyte space 31. Electrodes known per se can be used as the cathode, for example a tantalum mesh on which platinum black is applied as a catalyst material, or a cathode containing activated carbon. The cathode 38 is separated from the electrolyte space 31 by an asbestos cover layer 39 which is gas-tight in the electrolyte-soaked state. The cathode 38 is provided with a carbon fabric 40 on the side facing the gas space 33 for contacting and current collection. To operate the fuel element, gaseous hydrogen, for example, is supplied as fuel to the gas chamber 32 adjacent to the anode 34 via a line 41. The gas space 33 adjacent to the cathode 38 is supplied via a line 42, for example, with gaseous oxygen as an oxidizing agent. The electrolyte space 31 is supplied with the electrolyte, for example 2.5 molar sulfuric acid (2.5 m H ₂ SO,) via a line 43. The unused portions of the reaction gases or the electrolyte can be discharged from the fuel element via the lines 44, 45 and 46. For example, the electrolyte can have an operating temperature of about 70 ^° C.

Investigations on the phosphides IePp and GoP may also be mentioned by way of example from the above-mentioned group of metal phosphides. FePp gives in the oxidation of Hp at 7O ⁰ G in 2.5 m HpSO, with a polarization of 100 mV, measured against the reversible hydrogen potential, and a BeI-

p p

supply of about 100 mg / cm a current density over 15 mA / cm. PePp can also be used for anodic oxidation of formaldehyde. CoP gives under the same conditions,

see an occupancy density of 200 mg / cm, a current

density of about 30 mA / cm. The remaining metal phosphide catalysts, such as CuP ₂ , Au ₃ P ₅ , MnP ₅ , FeP ₅ IiP ₅ and N, have so far only been used at room temperature, where the activities are somewhat lower _{, similar to CoP 5.} It is particularly surprising that GuP _{2 shows} activity for the electrochemical oxidation of hydrogen.

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-H-

The metal phosphide catalyst can be used except in fuel cells according to the invention in other electrochemical cells - for electrosynthesis or electrolysis - as well as in Accumulators are used. When used in accumulators, the electrocatalyst according to the invention can in particular serve as electrode material for indicator and consumption electrodes in gas-tight accumulators. In such Accumulators must namely be taken to prevent that when charging the battery as a result Overcharging or gases formed during discharge exert an impermissibly high pressure. To achieve this, for example, the capacity of the positive electrode is selected to be greater than that of the negative electrode, so that when Initially overcharged hydrogen on the negative electrode

is developed (2H ₂ O + 2 e "^ - H ₂ + 2 0H ~). Man

now basically knows two options for an intervention. A so-called consumption electrode can be placed in the accumulator be installed, on which the hydrogen formed is converted electrochemically and brought back into solution in this way will. On the other hand, a so-called Use indicator electrode which is catalytically active and which is reversible when exposed to hydrogen Adjusts hydrogen potential. This potential, which is measured against a reference electrode, can then be used as a control signal can be used to stop charging. For both types of the named auxiliary electrodes, i.e. consumption electrodes and indicator electrodes, the metal phosphide catalyst of the invention can be used as the electrode material will.

13 claims
4 figures

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Claims (1)

YPA 72/7525 Claims (Γ) Metal phosphide catalyst for electrodes, in particular anodes, of electrochemical cells, in particular fuel elements with acidic electrolytes, characterized in that it contains at least one metal phosphide of one or more of the metals of the first and seventh subgroups and the metals of the iron group of the periodic table of elements contains a phosphorus content of the metal phosphide of at least 1 atom of phosphorus per metal atom. 2. Metal phosphide catalyst according to claim 1, characterized in that it contains at least one of the metal _{phosphides from the group CuP 2} , Au ₂ P ,, MnP ,, FeP ₂ , IeP, CoP ,, CoP, NiP, and NiP ₂ . 3. metal phosphide catalyst according to claim 2, characterized daäirch, that it contains CoP. 4. metal phosphide catalyst according to claim 1, characterized in that that the metal phosphide contains at least two different metals. 5. metal phosphide catalyst according to one of claims 1 to 4, characterized in that the catalyst material in powder form is. 6. metal phosphide catalyst according to claim 5, characterized in that that the powdery catalyst material has a grain diameter between 2 and 50 / u. 7. metal phosphide catalyst according to any one of claims 5 or 6, characterized in that the powdered catalyst material a plastic binder is mixed in. 3. Metal phosphide catalyst according to one or more of the Claims 1 to J, characterized in that it additionally 309841/1032 _Λ , Ίο VPA 72/7525 221619? ~ ¹⁶ " another catalytically active substance, in particular an organic semiconductor, tungsten carbide or a platinum metal, contains. 9. metal phosphide catalyst according to one or more of claims 1 to 8, characterized in that it is additionally contains an electrically conductive carrier material. 10. A method for producing a metal phosphide catalyst according to one or more of claims 1 to 9> characterized in that a powdered metal or a metal compound at elevated temperature in the presence of an additive remaining in the end product to form a large surface, in particular activated carbon, is made to react with phosphorus. 11. The method according to claim 10, characterized in that the metal by reduction of a metal salt, in particular a metal oxalate or a metal carbonate will. 12. The method according to claim 10, characterized in that a metal pyrophosphate is reduced with hydrogen Metal phosphide is represented by phosphidation in a metal phosphide with a higher phosphorus content is transferred. 13. Method according to one of Claims 10 to 12, characterized in that that another, soluble additive, especially sodium fluoride, is used and after completion of the catalyst is dissolved out of this. 30984 1/103 2