ELECTRODE MATERIAL, ELECTRODE, AND METHOD FOR HYDROGEN CHLORIDE ELECTROLYSIS

Abstract

An electrode material, an electrode and a process for hydrogen chloride electrolysis based on platinum metal as catalyst, in which the electrode material has a nanosize mixture of platinum particles and silver particles, is described.

Description

The invention relates to an electrode material based on platinum metal as catalyst, an electrode composed thereof and a process for hydrogen chloride electrolysis.

Starting from polyvinyl chloride (PVC) via foams through to medicaments and crop protection agents, chlorine chemistry contributes up to 60% of the sales of the German chemical industry. The high reactivity and selectivity of chlorine as reactant is usually exploited here, and the chlorine is obtained again as by-product or coproduct in the form of hydrochloric acid. Apart from other uses of hydrochloric acid (marketing, use in other processes), it can be electrically redissociated to recycle chlorine. In a further development of the classical electrolysis process, electrolyzers which avoid evolution of hydrogen on the cathode side and instead have oxygen-consuming cathodes at which oxygen-containing gases can be reduced are increasingly used nowadays for hydrogen chloride electrolysis. In this way, up to 30% of the required energy can be saved by reducing the electrolysis voltage. Although platinum is in principle outstanding by having the highest activity and selectivity for the reduction of oxygen, preference is given to using supported rhodium sulfide catalysts. The reason for this is the highly corrosive conditions under which the HCl electrolysis takes place, which lead to deactivation and dissolution of platinum. In view of the high raw materials price for rhodium and the lower activity compared to platinum, an improved catalyst based on platinum would be of great economic importance in the context of increasing energy consumption and increasingly scarce resources.

In the membrane electrolysis of hydrogen chloride, the electrode material is exposed to relatively harsh conditions. Thus, it has to withstand the corrosive chlorine-containing solution which cannot be completely held back from the cathode side by the polymer membrane and in the case of platinum as catalyst is reduced to chlorine over the platinum. In the case of platinum, the effect of chloride ions as catalyst poison has been adequately studied. Particularly in the case of a shutdown of the electrolysis plant (planned or unplanned due to bottlenecks in power supply), a sudden increase in potential at the platinum in the presence of dissolved chlorine and chloride ions can lead to a substantial loss of catalyst material due to dissolution of the platinum in the form of hexachloroplatinic acid and to deactivation of the remaining catalyst material (see J. R. Giallombardo, D. Czerwiec, E. S. De Castro, C. K. Shaikh, F. Gestermann, H.-D. Pinter, G. Speer, R. J. Allen. “Process for the electrolysis of technical-grade hydrochloric acid contaminated with organic substances using oxygen-consuming cathodes”, U.S. Pat. No. 6,402,930 and A. P. Yadav, A. Nishikata and T. Tsuru. Electrochim. Acta (2007), 52 [26], pages 7444-7452). However, regardless of any shutdown of the plant, a rise in the overvoltage for the reduction of oxygen over polycrystalline platinum catalysts can be observed in the presence of chloride ions (see T. J. Schmidt, U. A. Paulus, H. A. Gasteiger and R. J. Behm. J. Electroanal. Chem. (2001), 508 [1-2], pages 41-47).

To solve the problem of the low stability of the platinum catalyst for HCl electrolysis, novel, supported rhodium sulfide-based (Rh_xS_y) catalysts for HCl electrolysis which are virtually insensitive to many organic and inorganic compounds and do not suffer a loss of catalyst in the event of a shutdown of the electrolysis plant have been developed. An electrode material based on rhodium sulfide catalysts, an electrode and a corresponding HCl electrolysis process are subject matter of the international application WO 2002 018 675 A2. The electrode material described here is, owing to its greater chemical stability, used instead of electrode material based on platinum catalyst. WO 2002 018 675 A2 explicitly describes this advantage of the rhodium sulfide catalyst. However, its deficiency is the lower catalytic activity compared to platinum. In view of the comparatively high commercial price of rhodium and the lower catalytic activity compared to platinum, there continues to be a need for new and improved catalyst materials for electrodes, in particular cathodes of HCl electrolyzers. The successful further optimization would, moreover, be important not only for HCl electrolysis but also for other electrolysis processes such as chloralkali electrolysis. Even small improvements can achieve large effects in energy saving. Thus, each millivolt saved in the cell voltage in the chloralkali electrolysis could achieve an annual worldwide saving of 32 million kWh.

It is therefore an object of the present invention to provide novel and improved catalyst materials for HCl electrolysis, which should have a higher activity than the rhodium sulfide catalyst used at present while maintaining an equally high chemical stability under the conditions of the industrial HCl electrolysis.

A further object of the invention is to provide an electrode material and an electrode based thereon which avoids the disadvantages of the known electrodes and has a comparatively longer operating life in the HCl electrolysis.

The object is achieved by using an electrode material which is based on a mixture of nanoparticulate platinum metal and silver metal.

The invention provides an electrode material for hydrogen chloride electrolysis which is based on platinum metal as catalyst, characterized in that the electrode material has a nanosize mixture of platinum particles and silver particles, where platinum and silver have a particle diameter of essentially not more than 1 μm, preferably not more than 0.5 μm, particularly preferably not more than 0.1 μm.

The novel electrode material of the invention can be used either in supported form on a conductive inert support or in unsupported fowl.

The novel electrode material does not require any activation step before use and retains its full electrode catalytic activity in respect of the reduction of oxygen even in the presence of chloride ions. Furthermore, the novel electrode material is not dissolved by the complexing action of mixtures of aqueous hydrochloric acid and chlorine gas, so that no specific precautionary measures are necessary when shutting down the hydrochloric acid electrolyzers in which the electrode material is used.

To produce the novel gas diffusion electrodes, the novel electrode material is preferably applied to at least one side of a conductive sheet-like textile structure. The novel electrode material can be used either alone or together with a binder mixed with a conductive support material or supported on a conductive support material and combined with a binder. The binder can be hydrophobic or hydrophilic and the mixture can be applied to one or both sides of the sheet-like structure.

Preferred binders are fluoropolymers such as polytetrafluoroethylene (PTFE, commercially available, inter alia, under the name Teflon® (from DuPont)), polyvinylidene difluoride (PVDF), polymeric perfluorosulfonic acids (PFSA, obtainable, inter alia, under the name Nafion® (from DuPont)) or other proton-conducting ionomers known to those skilled in the art.

Electrode structures or base materials containing gas diffusion layers as are known from EP 0931857 and U.S. Pat. No. 4,293,396 and can be obtained, inter alia, under the name ELAT® (from BASF Fuel Cell Inc.) can typically be used.

The sheet-like structure can be a woven fabric or a nonwoven made of electrically conductive material or consist of a carbon cloth, carbon paper or any conductive metal mesh.

Examples of preferred support materials, in particular support materials having a large surface area, encompass graphite, various forms of carbon, in particular carbon nanotubes, and other finely divided supports, with carbon black being particularly preferred.

Such sheet-like structures coated with the novel electrode material can be used as gas diffusion cathodes which achieve a cell voltage and long life which have hitherto not been able to be achieved under conventional operating conditions. This applies particularly to the use of the electrode material in highly aggressive environments as occur in the electrolysis of hydrochloric acid as by-product.

Preference is given to an electrode material in which the weight ratio of platinum to silver is from 10:90 to 90:10, preferably from 30:70 to 70:30, particularly preferably from 40:60 to 60:40.

Further preference is given to an electrode material which is characterized in that the material additionally has particles composed of alloys of platinum and silver.

An advantageous preferred electrode material has platinum particles and silver particles and optionally alloy particles which have, independently of one another, an average particle diameter in the range from 1 nm to 100 nm, preferably from 2 nm to 50 nm and particularly preferably from 3 to 25 nm.

In particular, the platinum and silver particles can form agglomerates having an average agglomerate diameter of less than 100 μm, preferably less than 10 μm.

A particularly preferred electrode material is characterized in that the platinum and silver particles are obtained by simultaneous electrodeposition of platinum and silver, in particular by electrodeposition using a pulsed voltage, from platinum and silver salt solutions or melts, in particular from aqueous platinum and silver salt solutions, onto an electrically conductive support material.

The electrodeposition using a pulsed voltage is preferably carried out at an open-circuit voltage of from 0.4 to 0.8 V measured relative to a silver-silver chloride reference electrode in 3 molar potassium chloride solution, using voltage pulses in the range from −0.4 to −0.8 V and a pulse length in the range from 5 to 100 ms.

The invention further provides a chlorine-resistant electrode for electrochemical processes which has an electrode material based on a mixture of platinum and silver and can be installed as cathode in hydrogen chloride electrolysis.

A preferred chlorine-resistant electrode has the novel electrode material.

In one embodiment, the electrode is particularly preferably an oxygen-consuming cathode.

In the embodiment as oxygen-consuming cathode, the electrode is configured as gas diffusion electrode having an electrically conductive sheet-like textile structure as support, in particular a mesh, which is provided on at least one side with a catalyst which comprises the electrode material and optionally additionally comprises at least one binder containing fluorine compounds incorporated therein.

Preference is given to a gas diffusion electrode in which the conductive sheet-like structure is provided on one or both sides with a coating which comprises at least one fluoropolymer and at least one electrically conductive carbon material and is additionally coated on only one side with a mixture of the catalyst and at least one fluoropolymer.

In another embodiment, the electrode is particularly preferably a hydrogen-evolving cathode.

In the embodiment as hydrogen-evolving cathode, the electrode is, in particular, a graphite electrode in which the electrode material is applied as catalytically active coating to a graphite support.

The invention also provides a membrane-electrode assembly which comprises an ion-exchange membrane which is provided on at least one side with a catalyst comprising the electrode material of the invention.

The invention further provides for the use of the electrode of the invention or the membrane-electrode assembly of the invention for the electroreduction of oxygen.

The invention further provides an electrochemical cell having at least an anode chamber containing an anode and a cathode chamber containing a cathode, which are separated from one another by a separator, where the cathode is an electrode according to the invention.

The invention also provides an electrochemical cell having at least an anode chamber containing an anode and a cathode chamber containing a cathode, which are separated from one another by a separator, where the separator is configured as a membrane-electrode assembly according to the invention.

In a preferred embodiment of the electrochemical cell, the separator is an ion-exchange membrane or a diaphragm.

Particular preference is given to embodiments of the abovementioned types of electrochemical cells which are characterized in that the anode chamber can be supplied with aqueous hydrochloric acid and the cathode chamber can be supplied with an oxygen-containing gas or with aqueous hydrochloric acid.

The invention further provides a process for the electrolysis of an aqueous hydrochloric acid solution to form chlorine, characterized in that aqueous hydrochloric acid is fed into the anode chamber and an oxygen-containing gas is fed into the cathode chamber in a novel electrochemical cell of the abovementioned types while the cell is supplied with an electric direct current.

The invention is illustrated below with the aid of FIG. 1 and the examples which, however, do not restrict the invention.

In the figures:

FIG. 1 a+b show scanning electron micrographs of the glassy carbon surface after electrodeposition of silver and platinum,

FIG. 2 shows an energy-dispersive X-ray spectrum of the glassy carbon electrode coated with platinum-silver nanoparticles,

FIG. 3 schematically shows the flow cell for testing the stability of the glassy carbon electrode coated with platinum-silver nanoparticles,

FIG. 4 shows chronoamperograms of a glassy carbon electrode coated with platinum-silver nanoparticles and a platinum-coated glassy carbon electrode,

FIG. 5 a+b shows the stability of the platinum-silver-coated glassy carbon electrode compared to the platinum-coated glassy carbon electrode.

EXAMPLES

Example 1

Production of the Pt—Ag Electrodes of the Invention

The Pt—Ag electrodes were produced by simultaneous electrodeposition of platinum and silver from a 10 millimolar ethylenediamine solution (pH 11) which was 3 millimolar in hexachloroplatinic acid and 3 millimolar in silver nitrate onto a glassy carbon electrode (diameter 3 mm). Prior cleaning of the glassy carbon electrode was carried out by mechanical polishing using various Al₂O₃ suspensions (average particle diameter: 1 μm, 0.3 μm and 0.05 μm) on a polishing felt.

Electrodeposition was carried out in a three-electrode system under potentiostatic control at room temperature in a single-compartment cell from 1 ml of solution volume. Apart from the glassy carbon working electrode, a platinum wire was used as counterelectrode (CE) and a silver helix was used as reference electrode (RE). The pulse profile shown in Table 1 was selected for the deposition.

TABLE 1
Pulse profile for the simultaneous electrodeposition
of platinum and silver
	Potential [V] vs.
	Ag/AgCl (3M KCl)		Time [s]
	E1	+0.60	5
	E2	−0.25	0.005
	E3 (Pt—Ag)	−0.65	25

The scanning electron micrographs (SEM) in FIGS. 1a and 1b show that the pulse profile selected leads to deposition of nanoparticles on the glassy carbon surface.

The platinum-silver content of nanoparticles can be found to be 50:50 by means of energy-dispersive X-ray spectroscopy (EDX) (see spectrum in FIG. 2).

Example 2

As comparative material, a platinum-modified electrode was produced by electrodeposition of platinum onto a glassy carbon electrode (diameter: 3 mm). The deposition of platinum was carried out by a method analogous to the deposition of the platinum-silver nanoparticles in Example 1 from a 10 millimolar ethylenediamine solution (pH 11) which was 3 millimolar in hexachloroplatinic acid, at a potential E3 of −0.75 V (25 s).

Example 3

Stability Test on the Pt—Ag Electrode and the Pt Electrode

The glassy carbon electrode coated with platinum-silver nanoparticles from Example 1 was simultaneously tested in comparison with the glassy carbon electrode coated only with platinum from Example 2 to determine its stability toward chlorine and chloride ions in an electrochemical flow cell (see FIG. 3).

FIG. 3 schematically shows the flow cell for the stability test. In the electrolysis cell (left-hand cell in FIG. 3) there are two platinum disk auxiliary electrodes (Ø1 mm, spacing 4 mm) which are located opposite one another and at which chloride was oxidized to chlorine during the entire time of the experiment. This was achieved by application of an external voltage of 1.5 V, which was provided by a simple laboratory voltage source, between the two auxiliary electrodes. The auxiliary electrodes were polished in a manner analogous to the glassy carbon electrodes before each experiment. The measurement of the stability of the glassy carbon electrode coated with platinum-silver nanoparticles and of the glassy carbon electrode coated only with platinum was carried out chronoamperometrically in the electrolysis cell 2 (right-hand cell in FIG. 3) at a potential of −0.15 V vs. Ag/AgCl (3 molar KCl) at which oxygen is reduced at the working electrodes to be examined (WE 1 and WE 2), at room temperature. The actual measurement cell (electrolysis cell 2) has a volume of about 200 μl, and the catalyst-coated electrodes have a spacing of 4 mm and are opposite one another. As counterelectrode (CE), use is made of a stainless steel capillary through which the solution flows out from the cell, and an Ag/AgCl (3 molar KCl) electrode served as reference electrode (RE). Aqueous 0.4 molar hydrochloric acid is pumped through the two cells at a pumping rate of 28 ml/h. This is loaded with chlorine in electrolysis cell 1 and then goes into electrolysis cell 2 in which the actual stability test is carried out.

The application of the potentials to the two working electrodes was effected by means of an 8-fold potentiostat from CH Instruments. To simulate the shutdown of an industrial HCl electrolysis cell, the cell was operated for 30 minutes and the potential was then switched off by means of a relay for 1 minute. The procedure was repeated ten times with the application of the oxygen reduction potential being shortened to 12 minutes. The currents which flowed were recorded for both electrodes to be examined during the entire time of the experiment. The chronoamperogram obtained is shown in FIG. 4.

The evaluation of the chronoamperograms is shown in FIGS. 5a and 5b.

FIG. 5a) (at left) and b) (at right) show the stability of the platinum-silver-coated glassy carbon electrode compared to the platinum-coated glassy carbon electrode; the currents indicated in FIG. 5a) were recorded at the end of the 12 min of the oxygen reduction phase shortly before the electrolysis cell was switched off again. FIG. 5b) shows the measured oxygen reduction currents normalized to the respective initial reduction current (before the 1st switching-off).

After the first switching-off, the absolute value of the oxygen reduction current for the platinum-silver-coated glassy carbon electrode was already greater than that for the platinum-coated electrode. With an increasing number of switching-off operations, the absolute value of the reduction current for the electrode coated only with platinum decreased more and more, while in the case of the glassy carbon electrode coated with platinum-silver it decreased only slightly to a then constant value of over 90% of the initial reduction current. The activity of the glassy carbon electrode coated with platinum-silver thus proved to be stable to the switching-off operations while the electrode coated with platinum was shown to be unstable against the switching-off operations.

Claims

1.-21. (canceled)

22. An electrode material for hydrogen chloride electrolysis which is based on platinum metal as catalyst, wherein the electrode material has a nanosize mixture of platinum particles and silver particles, wherein the platinum and silver particles have a particle diameter of essentially not more than 1 μm.

23. The electrode material as claimed in claim 22, wherein the weight ratio of platinum to silver is from 10:90 to 90:10.

24. The electrode material as claimed in claim 22, wherein the electrode material additionally comprises particles comprising alloys of platinum and silver.

25. The electrode material as claimed claim 22, wherein the platinum particles, silver particles, and alloy particles have, independently of one another, an average particle diameter in the range of from 1 nm to 100 nm.

26. The electrode material as claimed in claim 22, wherein the platinum and silver particles form agglomerates having an average agglomerate diameter of less than 100 μm.

27. The electrode material as claimed in claim 22, wherein the platinum and silver particles are obtained by simultaneous electrodeposition of platinum and silver from platinum and silver salt solutions or melts onto an electrically conductive support material.

28. The electrode material as claimed in claim 27, wherein the electrodeposition is carried out using a pulsed voltage and is carried out at an open-circuit voltage of from 0.4 to 0.8 V measured relative to a silver-silver chloride reference electrode in 3 molar potassium chloride solution, using voltage pulses in the range from −0.4 to −0.8 V and a pulse length in the range from 5 to 100 ms.

29. A chlorine-resistant electrode for electrochemical processes which comprises an electrode material based on a mixture of platinum and silver, wherein the electrode can be installed as a cathode in hydrogen chloride electrolysis.

30. A chlorine-resistant electrode for electrochemical processes which comprises the electrode material as claimed in claim 22.

31. The electrode as claimed in claim 29, wherein the electrode is an oxygen-consuming cathode.

32. The electrode as claimed in claim 31, wherein the electrode is configured as a gas diffusion electrode having an electrically conductive sheet-like textile structure as support, wherein at least one side of the electrode comprises a catalyst which comprises the electrode material and optionally additionally comprises at least one binder comprising fluorine compounds incorporated therein.

33. The electrode as claimed in claim 32, wherein the conductive sheet-like structure is provided on one or both sides with a coating which comprises at least one fluoropolymer and at least one electrically conductive carbon material and is additionally coated on only one side with a mixture of the catalyst and at least one fluoropolymer.

34. The electrode as claimed in claim 29, wherein the electrode is a hydrogen-evolving cathode.

35. The electrode as claimed in claim 34, wherein the electrode is a graphite electrode in which the electrode material is applied as a catalytically active coating to a graphite support.

36. A membrane-electrode assembly which comprises an ion-exchange membrane which is provided on at least one side with a catalyst comprising the electrode material as claimed in claim 22.

37. An electrochemical cell comprising at least an anode chamber comprising an anode and a cathode chamber comprising a cathode, wherein the anode and the cathode are separated from one another by a separator, and wherein the cathode is the electrode as claimed in claim 29.

38. The electrochemical cell as claimed in claim 37, wherein the separator is an ion-exchange membrane or a diaphragm.

39. An electrochemical cell comprising at least an anode chamber comprising an anode and a cathode chamber comprising a cathode, wherein the anode and the cathode are separated from one another by a separator, and wherein the separator is configured as the membrane-electrode assembly as claimed in claim 36.

40. The electrochemical cell as claimed in claim 37, wherein the anode chamber can be supplied with aqueous hydrochloric acid and the cathode chamber can be supplied with an oxygen-containing gas or with an aqueous hydrochloric acid.

41. A process for the electrolysis of an aqueous hydrochloric acid solution to form chlorine, wherein aqueous hydrochloric acid is fed into the anode chamber and an oxygen-containing gas is fed into the cathode chamber in an electrochemical cell as claimed in claim 37 while the electrochemical cell is supplied with an electric direct current.