EP0126490B1 - Diaphragm based on nickel oxide and process for its manufacture - Google Patents

Abstract

The invention concerns a NiO-based ceramic oxide diaphragm for the alkaline water electrolysis. The diaphragm, in accordance with the invention, contains 0.5 to 10% by weight (estimated as Ti based on the oxide mass) of titanium oxide in the porous NiO layer. Diaphragms of this type are obtained, in particular, by the oxidative sintering of a mass of nickel powder which has been applied under pressure to a nickel support, especially one consisting of nickel wire gauze. In the process the titanium is in the form of titanium metal, titanium oxide or a titanium compound which is added to the initial nickel powder. The titanium is present in the form of its oxide after the oxidation sintering treatment. In an alternative embodiment of the process, an already sintered porous mass of nickel or nickel oxide can be impregnated with a titanium compound and calcined to convert the titanium compound to its oxide.

Description

The invention relates to a diaphragm based on nickel oxide for alkaline water electrolysis and to a method for producing the same.

Alkaline water electrolysis generally takes place at relatively low temperatures (below 90 ° C.), which must be chosen in hot KOH because of the low chemical resistance of the asbestos diaphragms that are usually used. These low temperatures are both thermodynamically and kinetically disadvantageous: This means that an unnecessarily high electrolysis voltage is required, and the entire process appears uneconomical for energy reasons.

For this reason, numerous attempts have recently been made to either improve asbestos resistance in hot KOH or to find other diaphragm materials.

Potassium silicate was added to the KOH electrolyte to reduce asbestos solubility in KOH (R.L. Vic et al in Hydrogen Energy Progress IV, 4th WHE Conference, June 13-17, 1982, California, pp. 129-140). It is clear that this measure cannot be considered as finally satisfactory.

The same authors also used a teflon-bound potassium hexatitanate diaphragm originally from Energy Res. Corp. has been developed (see also M.S. Casper, "Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques", Noyes Data Corp., Park Ridge, 1978, p. 190). However, this diaphragm is not particularly inexpensive, and the voltage drop resulting from the diaphragm is comparable to that of asbestos diaphragms (see M.S. Casper).

In the Int. J. Hydrogen Energy 8, (1983), pp. 81-83, describes another separator for alkaline water electrolysis, which consists of polysulfone-bound polyantimonic acid, which acts as an ion exchanger. This separator, which is still under development, is not yet available. A major disadvantage of the same is in any case its high electrical membrane resistance of 1.0 to 0'8p cm ² at room temperature.

• i) die Erzeugung eines geeigneten Oxidmaterials, wie Zr02, BaTi03, K₂Tis0₁₃ usw., das als Hauptkomponente der porösen Schicht wirksam ist; und
• ii) ein Zusammensintern der Pulver bei hohen Temperaturen zwischen 1300 bis 1700 °C.

It was therefore made more diaphragms with lower electrical resistance, such as. B. a diaphragm made of sintered oxide ceramics (J. Fischer, H. Hofmann, G. Luft, H. Wendt: Seminar "Hydrogen as Energy Vector", Commission European Comm., 3-4 October 1978, Brussels, S. 277-290). Although this diaphragm is characterized by very good resistance values (0.027 to 0.27 Q cm ² at 25 ° C), the manufacture is not easy and comprehensive

• i) the production of a suitable oxide material, such as Zr02, BaTi03, K ₂ Tis0 ₁₃ etc., which is effective as the main component of the porous layer; and
• ii) sintering the powders together at high temperatures between 1300 and 1700 ° C.

Porous metallic diaphragms made of sintered nickel have also been proposed (P. Perroud, G. Terrier: "Hydrogen Energy System", Proc. 2nd WHE Conference, Zurich 1978, p. 241). These have a very low electrical resistance and are also mechanically stable and inexpensive. The major disadvantage, however, is that this diaphragm, like the electrodes, is also electron-conductive and there is too great a risk of short-circuiting in a compact design.

In order to avoid this disruptive electron conductivity, porous nickel oxide diaphragms have been developed by the applicant (DE-OSes 29 27 566 and 30 31 064), which are caused by oxidation of sintered metal at elevated temperature (DE-OS 29 27 566) or more simply by oxidizing firing a carrier pressed nickel powder layer can be obtained (DE-OS 30 31 064). These NiO diaphragms have excellent properties as separators for alkaline water electrolysis.

The diaphragms obtained using the simplified manufacturing process have since been used repeatedly in a wide variety of electrolysis tests and have proven themselves very well. Their long-term stability in alkaline water electrolysis was checked, the longest test to date lasting over 8000 hours (at 120 ^° C.). Even after this time, the diaphragms were still intact. Thermodynamic considerations suggest, however, that these diaphragms can be reduced to metallic nickel after a certain sufficiently long time on the cathodic side either by the cathode itself or by the hydrogen formed. This thermodynamic effect is only opposed by a kinetic inhibition, which has to subside after a previously unknown time. This may be sufficient for the purposes of water electrolysis, but there is still a degree of uncertainty.

The following experiment shows that these sinks exist correctly:

A diaphragm manufactured according to DE-OS 30 31 064 was exposed to a hydrogen atmosphere at 200 ° C. A gradual reduction of the Ni0 to Ni was observed, which increased suddenly after 1500 hours, so that after 2000 hours the total NiO was completely reduced.

In the temperature range from 140 to 170 ° C., this reduction proceeds significantly more slowly, but it is also noticeable here, as can be seen from FIG. 1: after 2000 hours, 7% of the oxygen contained in the NiO is removed from the diaphragm. (The stabilization takes place after approx. 4500 h, whereby approx. 10% of the oxygen is removed).

Such a reductive attack Hydrogen are subject to ceramic diaphragms made of thermodynamically stable oxide compounds, such as. B. Zr0 ₂ , SaTi0 ₃ , K ₂ Ti _s 0 ₁₃ etc. ( _see above) not, but the production of such diaphragms is associated with the disadvantages mentioned above, in particular very high production temperatures, and they are in 10 N KOH increased temperature attacked with time.

• 2 Ni + O₂ → 2 Ni0

erst bei der Herstellung des Diaphragmas abläuft. Dadurch kommt es lokal zu einer beträchtlichen Temperaturerhöhung und die äußere Herstellungstemperatur kann lediglich bei 1000 °C liegen, was vorteilhaft ist. Dabei entfällt ferner - bedingt durch den Herstellungsprozeß (oxidierende Sinterung) - die Notwendigkeit, eine inerte Atmosphäre aufrechtzuerhalten, was ebenfalls eine deutliche Erleichterung bedeutet.The "in situ" NiO diaphragm according to DE-OS 30 31 064, on the other hand, is alkali-resistant and its manufacture not only starts from a cheap starting substance, but also offers the decisive technological advantage that the exothermic reaction

• 2 Ni + O ₂ → 2 Ni0

expires only when the diaphragm is manufactured. This results in a considerable temperature increase locally and the external manufacturing temperature can only be 1000 ° C., which is advantageous. Due to the manufacturing process (oxidizing sintering), there is also no need to maintain an inert atmosphere, which is also a significant relief.

The invention is therefore based on the object of improving the reduction stability of the nickel oxide diaphragms under the conditions of alkaline water electrolysis.

The nickel oxide-based diaphragm according to the invention developed for this purpose is characterized by a titanium oxide content in the nickel oxide mass corresponding to 0.5 to 10% by weight of Ti based on the total oxide mass of Ti0 ₂ and NiO.

The titanium oxide content of the porous oxide mass preferably corresponds to 1 to 5% by weight of Ti, in particular 2.5% by weight of Ti, in each case based on the sum of Ti0 ₂ and NiO, and the diaphragm expediently has a framework-forming lattice made of anoxidized nickel.

It has surprisingly been shown that the reduction stability of the NiO diaphragms was exceptionally increased if the nickel powder in the manufacture of the diaphragm according to the method of claim 5 TiO in amounts of 1 to 20% by weight (based on the sum of metallic nickel and Titanium dioxide) was added (which corresponds to an addition of 0.6 to 13.0% by weight of titanium, based on the sum of titanium and nickel).

The grain size of the admixed powder should be approximately comparable or smaller than that of the nickel powder in order to ensure a uniform distribution of the titanium over the oxide mass.

Instead of titanium oxide, titanium powder can also be admixed to the nickel powder mass in the manufacture of the diaphragm, or else as a compound, which transform into titanium oxide during the oxidizing sintering treatment. If necessary, a nickel oxide diaphragm which has already been produced can also be impregnated with a titanium compound which is converted into oxidized form by afterburning.

• Figur 1 Kurven für die Reduktionsanfälligkeit von Nickeloxiddiaphragmen in Wasserstoffatmosphäre bei Temperaturen von 140 bis 170° C;
• Figur 2 Kurven für den Langzeitgewichtsverlust von keramischen Diaphragmen in 10 N KOH bei 120°C und
• Figur 3 ein Fließbild, daß die einzelnen Verfahrensschritte bei einer Herstellung von erfindungsgemäßen Nickeloxiddiaphragmen wiedergibt.

The invention is explained in more detail below with the aid of examples, reference being made to the attached drawings; show it:

• Figure 1 curves for the susceptibility to reduction of nickel oxide diaphragms in a hydrogen atmosphere at temperatures of 140 to 170 ° C;
• Figure 2 curves for the long-term weight loss of ceramic diaphragms in 10 N KOH at 120 ° C and
• Figure 3 is a flow chart that shows the individual process steps in the manufacture of nickel oxide diaphragms according to the invention.

example 1

A ceramic diaphragm based on NiO was produced according to DE-OS 30 31 064 with the addition of Ti0 ₂ . This manufacture comprises the individual steps indicated in FIG. 3:

Commercial carbonyl nickel powder (INCD-255; grain size 2 to 3 µm) was mixed with 10% by weight (based on the powder mixture, ie Ni + Ti0 ₂ ) of commercially available TiO from Merck and the mixture was suspended in acetone and evenly distributed over a smooth surface. After the suspension medium had evaporated, the layer formed in this way was cold-rolled onto a Ni mesh (wire thickness 0.2 mm, mesh size 0.25 mm). The procedure was repeated to also provide the second side of the nickel mesh with a powder layer. The evenly distributed powder layer can also be obtained by conventional methods without any suspending agent. Furthermore, as indicated in FIG. 3, further nickel powder can be added to the mixture during the slurrying.

Finally, the assembly was sintered in air at 1050 ° C for about 20 minutes.

The advantageous physical properties of the diaphragm thus formed, such as electrical resistance, mechanical stability, porosity or thickness, were in no way deteriorated compared to those of diaphragms according to DE-OS 30 31 084.

The chemical stability was, however, greatly improved, as can be seen from FIGS. 1 and 2: The decrease in oxygen in a pure hydrogen atmosphere at 140 to 170 ° C. can no longer be measured during the first 2000 h, which indicates an enormously increased reduction stability. In comparison, z. B. a diaphragm made of pure NiO in 2000 hours. 7% of the oxygen and even a diaphragm stabilized with Al ₂ 0 ₃ additive still loses about 1.5% of the oxygen content in the same time. Analogously, the already excellent chemical stability in hot KOH is further increased: As FIG. 2 shows, the total weight loss is after 2000 hours in 10 N KOH at 120 ° C only 0.3%. In comparison, a pure NiO diaphragm loses 0.8%, a BaTi0 ₃ diaphragm 2% and a diaphragm mixed with 5% AI ₂ 0 ₃ 8% of the total weight, which is attributable to A1 ₂ 0 ₃ .

This positive effect of the addition of titanium oxide is noticeable from 1 to 2 wt.% Ti0 ₂ .

Example 2

Before the slurrying step, 8% by weight of metallic Ti (based on the powder mixture) of approximately the same grain size as Ni was added to the Ni powder. The subsequent production steps were the same as in Example 1. After the oxidizing sintering, both nickel and titanium were present in oxidic form. This diaphragm had the same properties as that of Example 1 with respect to the reducibility in H ₂ atmosphere.

Comparative example

50% TiO _{2 were} added to the Ni powder before slurrying. Otherwise, the preparation corresponded to that of Example 1. The diaphragm produced in this way showed a weight loss of 10% after only 500 hours in 10 N KOH at 120 ° C.

Thereupon, this experiment was stopped and it was found that NiO diaphragms produced with such a TiO ₂ admixture are unsuitable for alkaline water electrolysis, even if the reduction properties (measured as weight loss in a hydrogen atmosphere at 140 to 170 ° C) are very good and one Do not lag behind the diaphragm according to Example 1.

This negative effect of an excessively high addition of Ti0 ₂ is already noticeable from 20% by weight of Ti0 ₂ based on the sum of Ti0 ₂ and NiO.

Claims (5)

1. Diaphragm based on nickel oxide for alkaline water electrolysis, characterized by a titanium oxide content in the nickel oxide mass, corresponding to 0.5 to 10% by weight of Ti relative to the total oxide mass of Ti0₂ and NiO.

2. Diaphragm according to Claim 1, characterized by a titanium oxide content in the nickel oxide mass, corresponding to 1 to 5% by weight of Ti relative to the total oxide mass of Ti0₂ and NiO.

3. Diaphragm according to Claim 2, characterized by a titanium oxide content in the nickel oxide mass, corresponding to 2.5 % by weight of Ti relative to the total oxide mass of Ti0₂ and NiO.

4. Diaphragm according to one of the preceding claims, characterized by a lattice, providing a skeleton, of surface-oxidized nickel.

5. Process for the manufacture of a diaphragm according to one of the preceding claims, in which a pressure-compacted nickel powder layer on a carrier, especially a nickel net, is subjected to an oxidizing sinter treatment at an elevated temperature until the nickel powder is converted to a porous nickel oxide mass, characterized in that 0.6 to 13 % by weight of titanium (relative to the sum of titanium and nickel) in the form of metallic titanium, titanium oxide or a titanium compound is added in the most uniform distribution possible to the nickel powder mass which is to be sintered, or the nickel oxide diaphragm, obtained after the oxidizing sinter treatment, is impregnated with a quantity of titanium compound corresponding to the desired titanium content and is subjected to a final burning treatment for converting the titanium to Ti0₂, so that a diaphragm with a titanium content of 0.5 to 10 % by weight of Ti, relative to the total oxide mass of Ti0₂ and NiO, is obtained.

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