CA1067141A - Electrocatalyst containing noble metals and bismuth - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides an electrocatalyst comprising platinum, palladium and mixtures thereof and bismuth. The bismuth content is preferably between 0.5 and 70% by weight, based on the catalyst. Advanta-geously, the bismuth can be partially replaced by lead. The electrocatalyst is useful in anodes for fuel cells and for determining the concentration of sugars.

Description

~067141 The invention relates to an electrocatalyst based on-platinum and/or palladium, and to an electrochemical cell employing said catalyst. ~
Electrocatalysts are used particularly in electrochemical cells -such as fuel cells. Among those used for the longest time in such current generators are electrocatalysts containing noble metals such as palladium and platinum. These noble metals and also Raney metals such as Raney nickel and Raney platinum also serve particularly for reacting liquid fuels dissolv-ed in the electrolyte, such as glycol, glycerin, methanol, sucrose and for~ate. The electrocatalysts are generally incorporated in sintered or so-called supported electrodes or by coating screens, which then serve as screen electrodes. One disadvantage of these electrocatalysts is their relatively rapid aging, i.e., a decrease in the activity with time. Raney nickel shows, in addition, low activity and high sensitivity to corrosion.
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It is an object of the present invention to provide an electro-catalyst having increased activity and a low aging rate, which also is particularly suited for reacting liquid fuels in fuel cells.
According to the invention, there is provided an electrocatalyst for the electro-chemical reaction of hydroxyl group containing hydrocarbons, comprising a noble metal selected from the group consisting of platinum, ; : ' palladium and mixtures thereof and bismuth, wherein the amount of bismuth is fro~ about 0.5 to 70% by weight of said catalyst.
The electrocatalyst according to the invention is advantageously su~ted for use in fuel cells. Thus, in another aspect, the invention pro~ides a fuel cell containing an anode having an electrocatalyst, a cathode and an electrolyte for reacting liquid reactants dissolved in said electrolyte the lmprove~ent which comprises an electrocatalyst comprising a noble metal selected fro~ the group consisting of platinum, palladium and mixtures thereof and bismuth, wherein the amount of bismuth is from about 0.5 to 70%

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by weight of said catalyst. By means of this catalyst, liquid reactants ~ 30 which are dissolved in the electrolyte, i.e., fuels, can be reacted at high -~ current densities with negati~e potentials. As compared to pure platinum, the electrocatalyst according to the in~ention has considerably increased ` ~

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activity~ for instance, for the reaction of glycol. However, the present catalyst is also suited for reacting other fuels such as methanol, glycerin, sucrose and formate. High current densities can be obtained with the present electrocatalyst particularly if it is used in the form of screen electrodes, since screen electrodes have large surface areas and ensure good fuel transport, i.e., ~-' ~

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~067~4'1 a good fuel supply to the electrode. To particular ad~antage, electrodes ; consistlng of packets of nets with different mesh sizes can be employed. A
further important ad~antage of the electrocatalyst according to the invention is the aging rate, which is substantially reduced in comparison with platinum, for example.
;~ Moreover in the electrochemical cells, the present electrocatalyst can also be used to advantage for determining the concentration of hydrocarbons . , containing hydroxyl groups, particularly sugars. Thus, the invention further provides an electrochemical cell for determining the concentration of a sugar comprising a counter electrode and a working electrode separated by a diaphragm, said working electrode containing an electrocatalyst, and a `
'''`'t ' solution of a sugar surrounding said electrodes, the improvement comprising an electrocatalyst comprising a noble metal selected from the group consist-; ; ing of platinum, palladium and mixtures thereof and bismuth, wherein the amount of bismuth is from about 0.5 to 70% by weight of said catalyst. Tests ha~e shown that high con~ersion rates are obtained with this catalyst and ; that sucrose, for exa~ple, can be reacted with constant current density in a wide poten*ial range. A correspondingly constructed sugar sensor can ~ determine small sucrose cencentrations, i.e., from about 10 3 mol/l on i`: 20 ~corresponding to 0.034% by weight).
The bismuth content of the present electrocatalyst containing the noble metals is preferably between from about 0.5 to 70% by weight, based ~; on the total weight of catalyst. The electrocatalyst can contain to advant-age, intermetallic compounds of platinum and/or palladium with bismuth, ;; particularly the intermetallic compound PtBi. The compound PtBi has a bis-- muth content of 51.7% by weight. Another intermetallic compound is, for instance, Pd3Bi which has a bismuth content of 39.6% by weight.
In the electrocatalyst according to the invention, the bismuth may ad~antageously be partially replaced by lead. Such catalysts are likewise distinguished by a high activlty and the aglng resistance is generally adequate.

igure 1 is a graph of the results obtained from a screen electrode

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~067141 . .
: containing an electrocatalyst consisting of platinum and bismuth.
Figure 2 is a graph of the results obtained from an electrode ,. containing an,electrocatalyst consisting of platinum, palladium and bismuth.
Pigure 3 is a graph of the r~sults obtained from electrodes con-:' taining an electrocatalyst consisting of platinum, palladium, bismuth and ~i1 ' lead.
Pigure 4 is a graph of the results obtained in life tests of screenod , :~ ' , .
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~'~ r ~067141 electrodes.
The invention will be explained in further detail with reference to examples and figures.
For fabricating screen electrodes, nickel screen with a mesh size of 0.19 mm, a wire thickness of 0.13 mm and a screen thickness of ~
0.3 mm is used. These screens are degreased, for instance, by means of `
acetone, and treated with hydrochloric acid (about 18%) to remove the surface oxide layer. The metals are then precipitated without current on circular screens having a diameter of about 40 mm, i.e., an area of about 12 cm ~ For this purpose, the screens are dipped into an immersion solution containing appropriate metal salts.
For fabricating screen electrodes containing platinum and -bismuth, a solution is used which is prepared as follows: about 75 ml of an aqueous solution of hexachlor~platinic ~VI) acid H2(PtC16~ having a platinum content of 3.9% by weight is diluted with S~ HN03 to make 1 liter. From this solution, which has a platinum content of about 2.9 g, about 5 ml aliquots are removed, i.e., in amounts which contain about lS mg platinum.
Each of these platinum salt solutions is reacted with an aqueous solution of bismuth (III) nitrate with a known bismuth content and diluted with water to about 50 ml~ The nickel screens are then immersed in these solutions.
To prepare immersion solutions which contain palladium in - addition to bismuth and platinum, 42.5 ml hexachloroplatinic (VI)-acid solu-tion is brought with S % HN03 to 1 liter and 2.1 g PdC12 are dissolved therein. From this solutionJ which contains about 1.65 g platinum and 1.25 g , ........................................................................ ~, palladium, again ali~uots of about 5 ml, which contain about 15 mg of noble metals, are removed and reacted with bismuth ~III) nitrate solutions.
Immersion solutions, which contain lead in addition to platinum, palladium ~; and bismuth, are prepared in such a manner that in the nitric acid solution of hexachloroplatinic (VI) acid and palladium chloride, 7.5 g lead acetate j 30 Pb(CH3C00)2. 3H20 are further dissolved therein. From this solution, which contains about 1.65 g Pt. 1.25 g -.

1067~4i Pd and 4.1 g Pb, are then removed aliquots of about 2 ml having a total metal content (Pt+Pd~Pb) of about 15 mg and reacted with the bismuth salt solutionsO
The screens are immersed in the dipping solutions until the metals are completely precipitated. This occurs after about 5 to 10 minutes and is recognized by the fact that the originally yellow-brown solution becomes color-less or assumes a slightly greenish hue. Besides certain quantities of bismuth, there is present on screens having an area of 12 cm , 15 mg of metals i.e., Pt or Pt+Pd or Pt+Pd~Pb, corresponding to a coating of 1.25 mg/cm ~
Test results which were obtained with such screen electrodes in the reaction of glycol, are shown in Figures 1 to 3. On the ordinate is plotted the current density, i,in mA/cm and on the abscissa the potential e -in mV, as ~ measured against an Hg/HgO reference electrode in 6m KOH. The electrolytic ; liquid is 6m KOH which has a content of 2m glycol; the electrolyte temperature was about 60 G.
; Figure 1 shows measuring results obtained with electrodes which con-tained only platinum besides bismuth. Curves 10 to 13 correspond to electrodes with a composition as per Table 1.
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Curve Amount of catalyst (mg) Catalyst coating (mg/cm ) total Pt Bi total Pt Bi -15.0 15.0 - 1.250 1.25 11 15.2 15.0 0.2 1.267 1.25 0.017 12 15.4 15.0 0.4 1.283 1.25 0.033 13 15.8 15.0 o.8 1.317 1.25 o.o67 It can clearly be seen from Figure 1 that the activity of the electrocatalyst according to the invention increases with increasing bismuth content.
In Figure 2, measuring results are shown obtained with electrodes which contained platinum, palladium, and bismuth. Curves 20 to 25 denote .~
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electrodes having a composition corresponding to Table 2 below.
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Table 2 .

CurveAmount of catalyst (mg) Catalyst coating (mg/cm ) -total Pt~Pd Bi total Pt+Pd Bi :

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15.0 15.0 - 1.250 1.25 21 15.2 15.0 0.2 1.267 1.25 0.017 22 15.4 15.0 o.4 1.283 1.25 o.o33 . :
23 15.8 15.0 o.8 1.317 1.25 o.o67 -24 16.6 15.0 1.~ 1.383 1.25 0.133 18.2 15.0 3.2 1.517 1.25 0.267 From Figure 2, it can likewise be seen that the activity of the electrocatalyst according to the invention increases with increasing bismut~ content. Other-wise, there i9 no substantial difference between a catalyst containing platinum~
palladium and bismuth and a catalyst containing platinum and bismuth.
In Figure 3, measuring results are reproduced obtained on electrodes which contained, besides platinum, palladium and bismuth, also lead, i.e., on . :;
electrodes, in which the bismuth is partially replaced by lead. The composi-tion of the electrodes may be taken from the following Table 3.
Table 3 CurveAmount of catalyst (mg) Catalyst coating (mg/cm ) total Pt+Pd+Pb Bi total Pt+Pd~Pb Bi .
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15.4 lS.o o.4 10283 1.25 o.o33 31 15.2 15.0 002 1.267 1.25 0.017 32 15.1 15.0 0.1 1.258 1.25 o,oo8 33 15.0 15.0 1.250 1.25 As is seen from Curves 30 to 32 of Figure 3, the activity of the catalyst according to the invention increases with increasing lead content: with the ; _ 5 _ "
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lead value remaining constant, the activity increases with decreasing bismuth content. It can be seen from Curve 33 that electrodes, in which the bismuth is co~pletely replaced by lead, show even higher activity. Such electrodes, however, have a considerable disadvantage in that the aging resistance of the electrodes decreases with increasing lead content.
In Figure 4, life tests are shown, from which the different aging resistances can be seen. On the ordinate is plotted here the current density i in mA/cm for a constant electrode potential of -750 mV, as measured against an Hg/HgO reference electrode, plotted versus the time t in hours. As the electrolytic liquid served 6m ~OH, which had a 2-m glycol content; the electro-lyte temperature was 60 C. The electrodes used for the life tests each consist ; of five of the screens described above with an area of 12 cm which were stacked and clamped to form an electrode packet.
; Curve 40 shows the measuring results on an electrode containing Pt, Pd and Bi with ano~le metal content of 75 mg and a bismuth content of 14 mg.
The coating of this electrode corresponds approximately that of the electrode per Curve 25 in Figure 2. In Curve 41, the measuring results are shown which were obtained with an electrode containing Pt, Pd and Pb; the electrode had a no~lemetal content of 32.2 mg and a lead content of 42.8 mg, and thus corres-ponded approximately to the electrode per Curve 33 in Figure 3. Comparing Curves 40 and 41, it is found that the electrode, according to the invention, with the electrocatalyst (Pt+Pd+Bi) ages very much less than an electrode with platinum, palladium and lead. The current density of the bismuth-containing electrode, after 180 operating hours, is about eight times as large as that of the lead-containing electrode, although the latter even exhibits stronger . activity initially.
In Table 4, results of potentiodynamic measurements on smooth electrodes with an electrocatalyst according to the invention, consisting of platinum and bismuth, are reproduced. In the preparation of the electrocatalyst, platinum 106'7141 and bismuth in varying amounts were melted together at temperatures between about 1500 and 1800 C. After cooling, the electrodes were provided with . contacts and their surface was polished. The activity of the electrodes was determined by reaction of 0.2 m glycol in 6 m ~OH at 60 C. :
Table 4 Metal content (% by weight) Current density (mA/cm ) Pt Bi . :.
100 0 8,4

3 It can be seen from Table 4 that the activity of the Pt/Bi catalyst increased approximately up to a weight ratio Pt : Bi of 1 : 1, i.e., approximately up to a composition corresponding to the intermetallic compound PtBi.
As already set forth, the electrocatalyst according to the invention can also be used for sugar determination. Thus, anodic currents occur which : . :
20 are suitable for sucrose determination already with very low sucrose concentra-tions, for instance in potentiodynamic measurements with Pt/Bi catalysts.
If the present electrocatalyst is used in sugar sensors, a method and a measuring cell can be used, for example, which is taught in German Offenlegungsschrift 1 812 870. In this connection, a fixed diffusion zone is . :
~ employed, two electrodes are separated from each other by a diaphragm and a ';` critical diffusion current is determined, which serves to determine the con-centration. Such a procedure is possible because high reaction rates can be obtained with the electrocatalyst according to the inventionO As the reaction of the sucrose takes place in a wide potential range with constant current . .
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density, a measuring cell can also be used for the concentration determination, :`-which is known from the German Offenlegungsschrift 1 773 5530 This measuring :
cell contains likewise two electrodes, of which one has a polarization resis-tance which depends on the concentration of the dissolved reactant. However, arrangements can also be used which comprise three electrodes (reference, counter- and working electrode).

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrocatalyst for the electro-chemical reaction of hydroxyl group containing hydrocarbons, comprising a noble metal selected from the group consisting of platinum, palladium and mixtures thereof and bismuth, wherein the amount of bismuth is from about a . 5 to 70% by weight of said catalyst.

2. The electrocatalyst of claim 1 wherein said bismuth is present as an intermetallic compound with said noble metal.

3. The electrocatalyst of claim 2 wherein said inter-metallic compound is PtBi.

4. The electrocatalyst of claim 1 which further comprises lead.

5. In a fuel cell containing an anode having an electro-catalyst, a cathode and an electrolyte for reacting liquid reactants dissolved in said electrolyte, the improvement which comprises an electrocatalyst comprising a noble metal selected from the group consisting of platinum, palladium and mixtures thereof and bismuth, wherein the amount of bismuth is from about 0.5 to 70% by weight of said catalyst.

6. The fuel cell of claim 5 wherein said liquid reactant is glycol.

7. In an electrochemical cell for determining the con-centration of a sugar comprising a counter electrode and a work-ing electrode separated by a diaphragm, said working electrode containing an electrocatalyst, and a solution of a sugar surrounding said electrodes, the improvement comprising an electrocatalyst comprising a noble metal selected from the group consisting of platinum, palladium and mixtures thereof and bismuth, wherein the amount of bismuth is from about 0.5 to 70% by weight of said catalyst.

8. The electrochemical cell of claim 7 wherein said sugar is sucrose.