NO141419B - ELECTRODE FOR ELECTROCHEMICAL PROCESSES - Google Patents

Description

The invention relates to an electrode for electrochemical processes, comprising a base consisting of a passivable material, and a covering layer which at least partially covers this base, and which contains activating substances.

Within technology, numerous electrochemical processes have found their way in, e.g. for the production of chlorine and alkalis from salt solutions in mercury or diaphragm cells, from chlorates, hypo-chlorides and the like, for oxidation of organic substances, for desalination, e.g. of seawater, and for cathodic corrosion protection. For such electrochemical processes, it is known to use cathodes

and anodes of graphite or impregnated graphite. In this case, the graphite anodes are degraded by electrochemical reactions, so in order to maintain a constant distance between the electrodes, it becomes necessary from time to time to replace the anodes and eventually to replace them.

In addition, there are known anodes of passivable metals, such as e.g. titanium, zirconium, niobium, tantalum, tungsten, aluminium, iron, nickel, lead, bismuth, which are practically stable, and which retain their dimensions unchanged during electrolysis processes. The preferably oxidic passivation layers that form on the surface of such a metal anode give the anodes an excellent durability against corrosive attack, but at the same time cause, due to their high electrical resistance, a considerable increase in the voltage drop. It is known to avoid this disadvantage to provide metal anodes with a covering layer containing activating substances such as e.g. platinum metals, compounds of platinum metals, alone or together with oxides of base metals, such as manganese, lead, titanium or tantalum. Furthermore, coating with numerous compounds has been proposed, e.g. carbides, borides, sulphides, phosphides and mixed oxides.

Important criteria for the usability of a cover layer are its durability in the electrolyte in question, the erosion resistance and especially the adhesion to the base of the electrode, since poor quality of the adhesion is often the cause of the electrodes failing. Many methods are known to improve the adhesion strength, which is essentially determined by the nature of the coating method, the material composition of the cover layer and the nature of the surface to be coated. It is also known to add an extra intermediate layer between the base and cover layer as an "adhesion agent". However, a partial release of the cover layer from the base cannot be ruled out using the known forms of pairing of base and cover layer.

In addition, something that is also important for the usability of electrodes is the connection between the base of the electrode and the power supply rods, such as e.g. consists of titanium, and which in turn is electrically connected to a rectifier via power rails. The quality of the mechanical and electrical connection is not least determined by the weldability or solderability of the materials used to manufacture the electrode base and power supply rods.

From Norwegian patent application 74 0031 there is e.g. known an intermediate layer consisting of a titanium oxide TiO^. and by flame or plasma spraying an electrode base of metallic titanium is applied. Adhesion of the activators to the electrode base is indeed improved by the intermediate layer, but the effect is limited by the limited thickness of the intermediate layer, and in the event of damage to the layer, a TiC^ barrier layer is formed on the titanium base in the shortest possible time by anodic coupling surface, whereby the anodic voltage

fall rises.

A task for the invention consists in improving the adhesion of the cover layer to the base of the electrode in such a way as to completely avoid weakening of the electrode's electrochemical activity caused by partial dissolution of the cover layer. Furthermore, it is a task for the invention to improve the mechanical and electrical connection between the base of the electrode and power supply rods made of titanium, which in turn are connected via power rails to a rectifier.

According to the invention, these tasks are solved with an electrode

of the type mentioned at the outset, whose base consists of a titanium oxide TiOx, where x = 0.25 - 1.50. According to a preferred embodiment of the invention, x = 0.45 - 0.60. According to a further preferred embodiment, the porosity of the electrode's base is 20-50 volume percent and the average pore diameter 0.5-5 mm. According to the invention, the surface of the electrode's base facing away from the cover layer is provided with a layer of metallic sintered titanium to improve weldability and solderability.

To produce a base for an electrode according to the invention, titanium metal and titanium oxide powder are mixed in a ratio of 7:1 - 1:3, possibly after adding a binder, such as e.g. an aqueous solution of polyvinyl alcohol, the mixture pressed into plates, rods or other suitable electrode shapes, and the pressed pieces then sintered in an inert atmosphere in the temperature range from 900 to 1500°C.

Mixtures with a higher oxygen content are suitable for sintering at higher temperatures than with oxygen-poor ones. In order to improve the homogeneity of sintered TiO^ bodies, it may be advantageous to use a two-stage production method where pressed pieces produced by the above-mentioned method are first finely divided and ground down and the resulting powder, possibly after the addition of a pressing aid such as paraffin, wax, polyethylene, the polytetrafluoroethylene etc. is pressed into plates or rods. With the help of suitably designed pressing pistons, reinforcement ribs and/or recesses are suitably pressed into the plates which extend through the base of the electrode and serve as gas extraction channels. The front pieces are then placed under a protective gas such as argon, heated to a temperature of approx. 1200-1400°C.

By means of the mentioned one- or two-stage temperature treatment of the compressed mixture of Ti and TiC^ powder, essentially homogeneous TiO phases are formed which correspond to the respective stoichiometric composition, and whose lattices are considerably disturbed. Thus, there is e.g. in the range x = 0.6-1.25 a NaCl-type compound with fully occupied lattice sites, in the range x <.0.42 the o£-titanium lattice is expanded with embedded oxygen, and in the ranges x = 0.42-0 ,60 and x = 1.25-1.50, the base of the electrode consists of mixtures of the disturbed Ti and TiO phases and the TiO and Ti203~ phases, respectively.

According to a further favorable embodiment of the invention, the porosity of the electrode's base is 20-50%. To produce a porous base, sintered pressed pieces with composition TiOx with x = 0.25-1.50 are finely divided and by sieving isolated fractions with a grain size between 1 and 12 mm are pressed into plates, which then, e.g. in an argon atmosphere, heated to approx. 1200-1400°C.

The average pore diameter is suitably approx. 0.5 - 5 mm. The large surface of such a base makes it possible to apply larger currents to the electrode without damaging the covering layer. Further advantages lie in the numerous statistically evenly distributed pores with which the base of the electrode is permeated, and which act as gas extraction

channels, as well as the relatively small weight of a porous base.

For power supply, a is attached to the base of the electrode

or several titanium rods which in turn via power rails e.g. connected to a rectifier. For making the connection between power supply rods and base, the usual methods such as blow soldering and especially welding are less suitable for an electrode base of TiOx with x = 0.25-1.50, as even with careful handling it is not possible to avoid the occurrence of cracks in the solder layer or the weld seam and also in the base of the electrode itself, and the voltage drop due to this fault rises to unluckily high values during the operation of the electrode. welding

and the solderability of the electrode's base is improved according to the invention by applying a layer of titanium powder inserted with a binder, such as etherified cellulose, to a surface of the mold piece by sandblasting or by pressing, and this layer is then firmly connected to the TiO base of the electrode by sintering at a temperature of approx. 1200°C in argon atmosphere. According to a suitable method, the titanium layer is applied to the base of the electrode by flame or plasma spraying.

The electrodes can also be produced by pressing titanium sponge into plate-shaped pieces, providing a layer of a mixture of titanium

and rutile powder or also with a TiO powder and then sintered at a temperature of approx. 1100-1400 C. According to a preferred method for producing the electrode, a layer of TiO^ powder in a sink is covered with a layer of titanium powder, after which both layers are compressed with a pressure of 300-3000 kp/cm 2 , shaped and sintered.

The sintered base is then provided with a covering layer containing at least one metal from the group platinum, palladium, iridium, ruthenium, osmium, rhodium, gold or silver or compounds of these metals such as oxides, nitrides or sulphides. Suitable methods for applying the cover layer are e.g. precipitation from solutions, application of a suspension, galvanic precipitation, plasma spraying, flame spraying or pyrolytic deposition from gas phase. The cover layer, which is burned in by heating to approx. 300-600°C, should cover at least 5% of the surface of the electrode's base and have a thickness of approx. 0.5-10 um.

The covering layer of electrodes according to the invention is firmly anchored in the disturbed crystal lattice of the base substance, so that even after several times of heat hardening with subsequent quenching of the electrode, no dissolution of the layer or reduction in the electrochemical activity can be observed. Wear and tear of the cover layers under erosive conditions such as those where e.g. present in electrolysis cells with fast-flowing electrolytes, becomes extremely small. The cleaved pore-containing surface of the electrode's base is also significantly larger than the surface of a correspondingly dimensioned solid metal electrode, so a larger amount of activating substances can be applied there. unit area and the electrode can be applied with a higher current density without damage to the activating substances. A further advantage of the electrode according to the invention is that gas extraction channels, stiffening ribs and the like can be pressed in during the production of its base, so that no additional post-processing is required.

Electrodes according to the invention preferably consist of wood

layer, the first of which, facing the electrolyte, contains noble metals or compounds of noble metals, the second layer a titanium oxide TiO^ with x = 0.25-1.50 and the third layer titanium. The layers are mechanically inextricably connected to each other, with the middle layer essentially ensuring firm anchoring of the first layer with the electrode's base and the third layer ensuring the weldability of the electrode's base to current supply rods made of titanium. The electrode thus combines the advantages of a base of metallic titanium in terms of weldability, with the advantage of a base of TiOx in terms of firm bonding of the cover layer. The thickness of the TiO^ and Ti layers, which form the base, and the ratio between the thicknesses of the two layers are determined exclusively by their function, i.e. mechanical stability and weldability of the base as well as bonding of the cover layer. Preferably, the thickness ratio is approx. 10:1 - 1:10. Porosity and pore size distribution are variable and can be adapted to the respective desired operating conditions by changing the grain size of the powders used as well as the pressing and sintering conditions, e.g. for the design of suitable gas extraction ducts.

Electrodes according to the invention are suitable for electrolysis of any kind, e.g. for aqueous chloralkali electrolysis or electrolysis of hydrochloric acid and of water, and also for carrying out organic oxidation and reduction processes, as anodes for cathodic corrosion protection, for fuel cells and galvanic cells.

The invention will be elucidated in more detail by examples with reference to the drawing, which is a diagram of the specific Example 1 electrical resistance of titanium oxides TiOx.

Titanium powder with a grain size of .06/µm and rutile powder, .01/x were initially mixed in a fast-running paddle mixer, then added with 5 parts by weight of a 2% aqueous polyvinyl alcohol solution and then mixed further for 10 min. The ratio Ti-powder/Ti02_powder was 7:1 - 1:3. The mixtures were then pressed in a sinking press with a pressure of 2 Mp/cm 2 into cylindrical bodies with a diameter of 100 mm and a height of 50 mm, and these were first dried at a temperature of 105°C and then in argon heated to 250°C and sintered.

In fig. 1, the specific electrical resistance of the cylinders is listed as a function of the oxygen content. The resistance rises steadily from the practically oxygen-free titanium and passes at x = 0.25 a maximum and at x = 0.50 a minimum. In region I there is a 0(-t i-addition mixed crystal with oxygen packed in octahedral holes, in region III the compound TiO is stable and its lattice sites are completely occupied. The resistance increases in this region and passes a submaximum and a subminimum at x respectively =0.9 and x =1.0. In region II, which extends between x = 0.42 and x = 0.60, disordered O^-Ti and TiO phases appear next to each other. In the regions IV and V, where the resistance rises further, there are finally mixtures of TiO and Ti2°3 resP« only Ti20^.

After the measurement was carried out, the cylinders were provided by flame spraying with an average 5 µm thick platinum layer, the adhesion strength of which was tested by quenching the cylinders heated to 200°C in water of approx. 18°C. Cylinders of oxygen-free titanium, which were used for comparison, already showed local peeling of the cover layer after 3-5 times of quenching, while the first minor defects in cylinders with composition TiOx - 0.25<x<D.42 and 0.60 <x ^, 50 - was observed after more than ten times of quenching, and the coating layer on bodies with composition x = 0.42-0.60 remained flawless even after twenty times of quenching. A further advantage of bodies with an oxygen content of 0.42 to 0.60 is the relatively low specific electrical resistance, whereas bodies with x>1.50 are already less suitable for electrodes because of the high resistance.

Example 2

61.4 parts by weight titanium powder, grain size ^), 06 /om, and 38.6 parts by weight rutile powder, grain size <J.01 /im - molar ratio approx. 8:3 - after the addition of 5 parts by weight of a 2% aqueous polyvinyl alcohol solution, was mixed for 10 minutes in a high-speed mixer and then pressed with a pressure of approx. 50 kp/cm 2 for cylindrical bodies with a diameter of 50 mm. After drying at a temperature of 105°C, these press pieces were then heated to 1250°C in an argon atmosphere over the course of 4 hours and then finely divided in a jaw crusher and ground down to a grain size of 0.06 µm in a swing mill. The powder was brittle with a color like gray cast iron and a composition of Tib^

100 parts by weight of the powder were then added with 5 parts by weight of a 10% solution of hard paraffin in toluene, mixed for 5 minutes in a

2 vortex mixers and pressed in a sinking press with a pressure of 2.5 Mp/cm into plates which on one side had ribs and cylindrical recesses with

diameter 2.5 mm and otherwise the dimensions were 350 x 450 x 10 mm. The plates were then dried at 110°C and heated to 1300°C in an argon atmosphere in a penetration furnace. The length of stay was 3 hours. The electrical resistance of the densely sintered sheets with a metallic luster amounts to approx. l.8Amm 2/m and the available pore volume approx. 15%.

The plates, which are intended as anode bases for chloralkali electrolysis cells, were smeared on the back with alcoholic solutions of 10 mol percent RuCl^t^O^ 5 and 10 mol percent H2Pt Clg and heated to 700°C in an argon atmosphere to burn in the cover layer. After cooling, the plates were smeared with an alcoholic solution of 25 mole percent RuCl3 (H20)15 and then heated to 650°C in water vapor-saturated air. The adherent, dark gray to black cover layer contains approx.

2 1.4 mg/cm precious metals.

The plates were tested as anodes in an alkali chloride amalgam cell. The electrolyte contained approx. 300 g/l Na Cl, the temperature was 80°C and the electrode distance 2 mm. The plates were applied with current densities of 10,000 and 20,000 A/m 2 respectively and then examined microscopically for changes in the covering layer. Damage or loss in the tire layer could not be determined. The anode potential measured with the Haber-Luggin capillary was 1.33 V referred to the normal-hydrogen electrode and likewise remained unchanged.

Example 3

37.5 parts by weight titanium powder and 62.5 rutile powder - molar ratio approx. 1:1 - was, as described in example 2, mixed with 5 parts of an aqueous polyvinyl alcohol solution, pressed, dried and then heated to 1300°C in an argon atmosphere. The press pieces with a molar ratio of Ti:0 as 1:1 were crushed, and the fraction 2 to 8 mm was sieved, inserted with a 5% solution of a montan wax in benzene, mixed, and then pressed with a pressure of 1.5 Mp/cm 2 for plates with the dimensions 300 x 200 x 8 mm. At the same time, a ribbed pattern was emphasized on the surface. The plates were then sintered at a temperature of 1250°C in a pure argon atmosphere for three hours. The pore volume of the boards was approx. 40%, average pore diameter approx. 2 mm. The plates were then provided by flame spraying with a 0.9 µm thick equimolecular platinum-iridium cover layer and heated in argon to 700°C to burn in the layer.

The plates were tested as anodes in a diaphragm test cell for the production of chlorine and caustic soda at a current density of 6 kA/m and an electrolyte temperature of 70°C. The loss of precious metal was less than 0.1 g per tonnes of chlorine produced.

I -r i ii'

Example 4

61.4 parts by weight of titanium powder, grain size ^),06 /im, and 38.6 parts by weight of rutile powder, grain size ^),01/im - molar ratio approx. 8:3 - after the addition of 5 parts by weight of a 2% aqueous polyvinyl alcohol solution, was mixed for ten minutes in a high-speed mixer and then pressed on a sinking press with a pressure of approx. 50 kp/cm 2 for cylindrical bodies with a diameter of 50 mm. After drying at 105°C, the press pieces were in

in the course of four hours heated in an argon atmosphere to 1250°C and then crushed in a jaw crusher and ground to a grain size of ^0.06/im in a swing mill. The powder was brittle with a color like gray castings and a composition of TiOg. It was then brought into a sink and covered with a layer

of titanium powder with grain size ^),1 mm. The powder layers were then pressed into plates with dimensions 350 x 450 x 10 mm and with ribs and cylindrical recesses with a diameter of 2.5 mm on one side. The TiO^ sides of the plates were wiped off with an acidic alcoholic solution

10 mole percent RuCl_ (H_0) .. c and 10 mole percent H.PtCl-, dried at 110 C and then ground through a furnace where it was heated to 1300°C

in an argon atom atmosphere and with a residence time of 3 h. After cooling, the plates were coated with an alcoholic solution of 25 mole percent RuC<l>3 (H^O)1 5 and then heated in steam-saturated air to 650°C.

At a current density of 10 kA/m 2 and a k-value of 0.09 V m 7/kA, the voltage drop on a horizontal chloralkali cell with a mercury cathode and an anode amounted to 4.0 to 4.1 V. Under the same conditions, the voltage drop on a cell with an anode consisting of a series of parallel-placed vertical titanium ribbons 4.25 to 4.30 V.

Claims (4)

1. Electrode for electrochemical processes, with a base of a porous, passivable material and with a cover layer that contains activating substances and at least partially covers this base, characterized in that the base of the electrode is made of a titanium oxide TiOx where x = 0.25 - 1.50. 2. Electrode as specified in claim 1, characterized in that x = 0.42 - 0.6. 3. Electrode as specified in claim 1 or 2, characterized in that the pore portion of the electrode's base is 20 - 50 percent by volume, and that the average pore diameter is 0.5 - 5 mm. 4. Electrode as specified in one of claims 1-3, characterized in that the surface of the electrode's base facing away from the cover layer is provided with a layer of sintered titanium.