GB2083837A - Manufacture of electrode with manganese dioxide coating, valve metal base, intermediate semiconducting layer - Google Patents

Abstract

A mixed oxide interface is provided between an eg. titanium base and the outer manganese dioxide coating by means of titanium from the base and noble metal from a solution containing a predetermined amount of HCl which attacks the titanium base surface. Slow drying provides a metal chloride mixture which is thermally converted to said mixed oxide of titanium and noble metal in a given ratio, whereby to protect the titanium base from oxidation. An outer coating of manganese dioxide is electroplated on the mixed oxide layer so as to provide an inexpensive electrode with improved resistance to oxidation. This electrode can be used for various processes where a high resistance to oxidation is required, e.g. for metal electrowinning processes.

Description

SPECIFICATION Manufacture of electrodes with a manganese dioxide coating, a valve metal base, and an intermediate semi-conducting layer FIELD OF THE INVENTION The invention generally relates to dimensionally stable catalytic electrodes, and more particularly to the manufacture of anodes which comprise a manganese dioxide coating electroplated on a valve metal base provided with an intermediate semi-conducting protective layer resistant to oxidative conditions.

BACKGROUND OF THE INVENTION Dimensionally stable anodes having a titanium base coated with a solid solution in the form a mixed oxide of titanium and ruthenium as described for example in U.K. Patent specification 1,195,871, have found widespread use since about 10 years for the electrolytic production of chlorine throughout the world. Their low power requirements and high durability largely offset their cost for such use. Such anodes are nevertheless subject to more or less rapid passivation in the presence of anodically evolved oxygen, thereby restricting the usefulness as anodes for reactions involving the release of oxygen.

U.S. Patent Reissue 29419 relates to an anode for electrowinning metals, which comprises a coating of ruthenium dioxide finely dispersed in an organic polymer matrix serving as a binder to provide mechanical support, adherence to the protection of the underlying substrates. In order to provide the required electrocatalytic properties, such a composite coating may include the electrocatalyst (Ru02) in a weight ratio of 6:1 to 1:1 with respect to the inert organic polymer.

The electrocatalyst must in this case be prepared in the form of extremely fine particles with a size less than 0.1 micron, applied together with the polymer and dispersed therein as uniformly as possible so as to form 85 to 50 weight percent of the coating.

The electrical conductivity and catalytic activity of such a composite coating will thus essentially depend on the amount of electrocatalyst, therein, as well as on its particle size and distribution throughout the inert organic polymer matrix. Electrocatalysts comprising platinum group metals must be used in as small amounts as possible and be generally combined with cheaper electrode materials which are more or less inert, but in most cases are catalytically inactive and also poor electrical conductors.

Various cheaper materials with potentially interesting properties may moreover be unsuitable for the industrial manufacture of electrodes.

Iridium dioxide and rhodium dioxide are suitable noble metal electrocatalysts for oxygen evolution but must also be used in minimum amounts to provide electrodes of acceptable cost.

Manganese dioxide is of particular potential interest as an inexpensive electrocatalyst for oxygen evolution, but it has nevertheless proven difficult to fabricate satisfactory anodes from this material.

The brittleness and relatively low conductivity of manganese dioxide make it unsuitable for manufacturing massive electrodes consisting of this material.

Thus, various proposals have been made to deposit manganese dioxide, by electroplating or thermal decomposition of a manganese salt, on an electrode base made of a valve metal, namely titanium.

The valve metal base is nevertheless in this case subject to oxidation by manganese dioxide deposited thereon, which undergoes reduction to a non-conductive manganese oxide thus rendering it unsuitable as an electrocatalyst. The valve metal of the electrode base is moreover subject to passivation in use, due to anodically released oxygen, as already mentioned.

It is moreover important to provide a catalytic anode surface which is as smooth as possible when it is used in electrolytes contaminated with impurities such as manganese which tend to form non-conducting anodic deposits leading to a progressive rise of the anode potential, and thereby requiring more or less frequent interruptions of cell operation for the removal of such deposits.

The fabrication of satisfactory electrocatalytic coatings of manganese dioxide on valve metal substrate remains problematic. The state of the art in this connection may be illustrated for example by U.S. Patents 4,028,215 and 4,125.449 which provides an electrode with a semiconducting intermediate coating formed of tin and antimony oxides, between a valve metal base and a manganese dioxide top coating.

An anode comprising a titanium base provided with a protective layer of titanium nitride and an outer coating of manganese dioxide is moreover described in U.S. Patent 4,060,476.

U.S. Patents No 4072 586 and No 4 180445, Japanese published patent application No 11753/1980 and an article published in Electrochimica Acta, 1978, vol. 2.3, pp 1978 moreover relate to anodes comprising a valve metal base, an intermediate oxide coating comprising platinum group metal and an outer manganese oxide coating.

DISCLOSURE OF THE INVENTION An object of the invention is to provide a simple, reproducible method of manufacturing improved catalytic electrodes comprising a manganese dioxide coating, a valve metal base and an intermediate electro-conducting protective layer.

Another object of the invention is to provide such electrodes with an intermediate protective layer wherein noble metals are incorporated as economically as possible, i.e. in an optimum manner such as to ensure high stability and resistance to anodically evolved oxygen, good protection of the underlying valve metal base from oxidation, high adherence of the manganese dioxide coating to the valve metal base, good electrical contact with low electrical resistance so as to avoid any excessive potential drop and to ensure electrodeposition of a manganese dioxide coating providing satisfactory performance of the catalytic electrode for long periods of operation. more particularly as an anode for oxygen evolution.

The invention provides a method of manufacturing catalytic electrode as set forth in the claims.

The valve metal base used in accordance with the invention may be any suitable electrode base serving as a support for a catalytic coating and consisting essentialiy of a valve metal such as titanium, zirconium, tantalum, niobium, or of a valve-metal based alloy. or at least comprising such a valve metal or alloy at the surface of the base to provide a valve metal substrate for forming the mixed oxide layer according to the invention. A very thin mixed oxide layer is formed gradually under strictly conteolleJ conditions from the surface of the valve metal base which provides a valve metal component of the mixed oxide. while the solution applied to the base provides a noble metal component of the mixed oxide.

The solution applied to the base must contain a sufficient amount of hydrogen chloride to attack the base surface, to thereby convert the valve metal thereon to a corresponding chloride mixed with the noble metal chloride applied with the solution. and to thereby provide a chloride mixture for thermal conversion to the mixed oxide.

The amount of valve metal from the base which is converted to chloride will evidently depend on one hand on the HCI concentration in said solution and on the other hand on the time available for such a conversion. Consequently, the applied solution should be dried slowly without any significant elevation of temperature, so as to provide the time necessary for conversion to the valve metal chloride.

In addition, the applied solution should properly wet the base surface in order to ensure said conversion of the valve metal thereon.

The invention was successfully carried out with isopropylaicohol as a solvent for the applied solution, although other alcohol solvents such as ethanol and butanol were likewise used successfully. On the other hand. water is apparently unsuitable as a solvent for carrying out the invention. This may be due to insufficient wetting of the valve metal by the water based solution and/or to too rapid evaporation of the hydrochloric acid.

The HCI concentration required to provide a given molar ratio of valve metal to noble metal converted to a mixed oxide may be theoretically calculated.

However, some excess HCI will generally be provided to ensure the required conversion.

Moreover, in order to be able to ensure the formation of a mixed oxide integrated in the valve metal base surface in accordance with the invention, the molar ratio of HCI to noble metal chloride present in the applied solution must be kept within given ranges. This molar ratio will depend in each case on the desired ratio of valve metal to noble metal in the mixed oxide to be formed, as well as on the amount of valve metal which can be effectively converted to chloride in practice, and more particularly on the amount of HCI which can reach and effectively attack the valve metal surface.

It may moreover be noted in this connection that the number of layers of solution which may be applied and thermally converted to a mixed oxide according to the invention will depend on various factors, and more especially on the concentration of noble metal in the applied solution in each case The invention was carried out successfully with iridium chloride (IrCI3) dissolved in isopropyl alcohol in a concentration corresponding to about 7 grams of iridium metal per liter. Mixed oxides were also formed according to the invention with iridium concentrations from 3.5 g/l to 35 g!J.

The noble !i:rrrri chloride concentration may nevertheless be selected in a still broader concentraton range from about 1 x 10-2 to about 25 X 10-2 mole per liter of solution, although the narrower ranges of 2 x 10-2 to 10 X 10-2 and especially 2.5 X 10-2 to 7.5 x 10 moles per liter are preferred to ensure satisfactory mixed oxide formation in accordance with the invention.

However, as already indicated above, the concentration of HCI should in each case be selected, according to the concentration of the noble metal chloride present in the applied solution, so that their molar ratio lies within a given range to provide satisfactory formation of a mixed oxide. The HCI concentration may thus also be selected from a relatively broad range, from about 14 x 10-2 to about 3 mole HCI per liter, but the selected value will depend in each case on the selected concentration of noble metal chloride present in the solution applied when carrying out the invention.

The previously mentioned range of molar ratio of HCI to noble metal chloride concentration may extend between 1:1 and 100:1, and preferably between 3:1 and 30:1 when carrying out the present invention, but both concentrations must be high or low at the same time.

It has been established that the formation of mixed oxide according to the invention is not possible when the HCI concentration is exceedingly high, e.g. 200 g/l and the noble metal concentration is exceedingly low, e.g. 2-3 g/l, and that in this case no useful results are achieved with regard to providing an electrode with adequate long term performance.

On the other hand, as may be seen from the examples given further on, excellant performance as an anode for oxygen evolution is achieved when said molar ratio is selected within said given range in accordance with the invention.

Large electrodes for industrial use could moreover be manufactured without difficulty in accordance with the invention, namely by following the special teachings of the invention for forming a mixed oxide by means of, on one hand, valve metal provided by the electrode base itself, and on the other hand, noble metal provided by the applied solution.

As may be seen further on, the mixed oxide which is thus "grown" from the valve metal base and thereby completely integrated in the surface of the electrode base can provide excellent protection of the valve metal base by means of a relatively low noble metal loading, while presenting a practically negligible electrical resistance, so as to thereby provide in a quite simple and economical manner, an excellant electro-conducting intermediate substrate for the subsequent electrodeposition of manganese dioxide, which can thereby be carried out efficiently under improved conditions.

This intermediate mixed oxide substrate thus formed in accordance with the special teachings of the invention moreover provides not only excellent protection and a low potential drop, but also excellent bonding of the electroplated manganese dioxide coating to the valve metal base.

This excellent bond in turn provides an improvement of the quality and performance of the resulting electroplated manganese dioxide coating, and hence a considerable improvement of the long-term performance and service life of the electrode.

The chloride mixture obtained according to the invention by applying a solution containing hydrogen chloride and noble metal chloride in given proportions, and slowly drying the applied solution, is converted to a mixed oxide by subjecting said mixture to heat treatment in an oxidizing atmosphere at relatively high temperature lying in the range from 400"C to 600"C, more particularly in the range from about 450"C to about 520"C.

This heat treatment provided satisfactory conversion to a mixed oxide at a temperature of about 480"C for about 5-10 minutes in air flow, but treatment at a lower temperature may require a longer time and vice-versa.

On the other hand, heat treatment at a relatively low temperature of 400"C for a relatively long period of 1 hour did not provide satisfactory results.

The duration and temperature of said heat treatment should thus be mutually adapted in each case so as to ensure satisfactory conversion of said chloride mixture to a mixed oxide, while avoiding an undesirable oxidation of the underlying valve metal of the base.

In accordance with the invention, the sequence of steps, comprising: applying a solution of suitable, controlled composition, slowly drying, and controlled heat treatment for conversion of the chloride mixture to a mixed oxide should be carried out cyclically several times, namely at least twice, so as to gradually form a mixed oxide of adequate thickness, containing a sufficient amount of noble metal.

The first layer of mixed oxide thus formed will be relatively porous, thus allowing the solution subsequently applied to penetrate this first porous layer, to thereby attack the underlying valve metal for further conversion to a corresponding valve metal chloride, whereby to additionally form said chloride mixture for further conversion to a mixed oxide, which is thus formed partly within the pores of the first layer.

The porosity of the resulting mixed oxide layer is thus gradually reduced each time the said cycle of steps for forming a mixed oxide is repeated, until no more valve metal from the base can be effectively converted to chloride and hence to the mixed oxide.

Any further repetition of said cycle of steps would no longer allow the formation of a mixed oxide according to the invention, and would moreover be undesirable, since it would lead to the formation of a simple noble metal oxide which, as is well kown, is much less stable than a mixed oxide comprising a significant amount of valve metal.

An extemely stable, homogeneous relatively compact and impermeable electro-conducting mixed oxide may thus be gradually obtained from the valve metal base by cyclically repeating a sequence of simple, well-controlled steps in accordance with the invention.

However, as aleady indicated, the amount of mixed oxide which can thus be formed according to the invention is limited in each case, while further application of the solution should in fact be avoided since it would lead to undesirable formation of a less stable oxide.

The number of layers of solution which can be effectively applied so as to allow formation of a mixed oxide according to the invention will largely depend on the noble metal concentration in the solution applied in each case.

Thus, for example, a solution comprising IrCI3 corresponding to 7 9 Ir/liter of solution provided excellent results when said sequence of steps for forming a mixed oxide were repeated 4 times according to the invention.

However, the number of repetitions of said sequence of steps may be increased up to 20 times or possibly more, especially in such cases where the noble metal concentration in the applied solution is significantly reduced so as to approach the lower limit of the corresponding concentration range given above.

On the other hand, when relatively high noble metal concentrations are used, the number of times the solution is applied will have to be reduced to e.g. between 2 and 4, in order to allow formation of a mixed oxide only, as well as to avoid a prohibitively high loading of the valve metal base surface with noble metal in the form of a relatively unstable mixed oxide comprising a reduced proportion of valve metal. It may further be noted that the solution for forming a mixed oxide according to the invention may be applied by any suitable means such as a brush or spraying device for example.

As regards the amount (v) of solution which may be applied each time, good results were obtained according to the invention by applying 10-20 ml of said solution per square meter of the valve metal base surface. Moreover 50 ml/m2 could also be applied by spraying, while as little as 5 ml/m2 may possibly be applied.

The total loading (L) of noble metal incorporated in the form of a mixed oxide per unit area of the surface of the valve metal base, will evidently be proportional to the noble metal concentration (CNM) in the applied solution, the number (N) of times it is applied, and the amount of solution (v ml/m2) applied each time.

Good results were obtained by means of the invention with noble metal loadings (L) corresponding to 0.5-1 gram noble metal in the form of a mixed oxide per unit surface area of the valve metal base.

Although a satisfactory result may be achieved with a noble metal loading somewhat lower than 0.5 g/m2, this value is already so low that a further reduction would hardly provide any further significant economic advantage.

On the other hand, it was found that extremely low noble metal loadings of about 0.2 g/m2 did not-provide satisfactory results with a reasonable number (N) of applications of the solution.

It was moreover found that the noble metal loading may be somewhat increased above 1 g/m2, for exampe to 1.2 g/m2 or possibly up to about 1.5 g/m2 in some cases, if desired.

It is thus apparent from the foregoing explanations that a reasonable compromise should be found for the above-mentioned parameters within the corresponding indicated ranges, so as to provide the best result according to the invention, depending on the particular electrode requirements in each case. Thus, for example, the number of applications of the solution, followed each time by drying and heat treatment should evidently be kept within reasonable limits. This number of applications should on one hand be increased to provide all of the advantages of the invention, whereas an excessively high number of applications is undesirable as beeing too onerous, while also not fully providing all of the advantages of the invention.

The following examples serve to illustrate the invention: EXAMPLE 1 Titanium base sheets (10 X 2 cm) were degreased, rinsed in water, dried and etched for 30 minutes in oxalic acid. A solutiom S6 comprising: 10 ml isopropanol (IPA), 0.06 ml HCI, and 0.16 g IrCI3 aq. (48 Wt % Ir) was then applied with a brush to the pretreated titanium sheet samples and allowed to dry slowly in air.

A heat treatment was then effected at 480'C for 7 minutes in an air flow of 60 I/h in order to produce a mixed oxide of titanium and iridium at the base surface.

This sequence of steps was repeated several times so as to gradually form a semi-conducting mixed oxide layer comprising titanium from the base and a given amount of iridium and to thereby provide a mixed oxide intermediate substrate for electroplating. This mixed oxide substrate was then topcoated with a catalytic layer by anodically depositing manganese dioxide at a current density of 1.5 to 20 A/cm2 for respectively 20 and 1.5 hours, from a 2 M manganese nitrate bath at a temperature of 90"-95"C.

The MnO, topcoating was finally heat treated at 400'C for 20 minutes in an air flow at 60 I/h to improve the electrode performance.

The resulting catalytic electrode samples coated with MnO2 were finally subjected to accelerated testing as an oxygen-evolving anode, at a fixed anode current density (ACD) lying in the range of 500-7500 A/m2, in an electrolytic cell containing H2SO4 at 150 g/l at 45 -55 C,.

The initial anode potential of each sample tested was determined with respect to a normal hydrogen electrode (V/NHE), but without correction for ohmic drop. The final potential at the end of the test period was also determined except where anode failure occurred.

Table 1 shows the references as well as corresponding data relating to the preparation and test conditions of electrode samples prepared as described above.

The second column in Table 1 relating to preparation respectively indicates at the top the number of applications and type of solution applied S6 and at the bottom the duration (h) and current density (A/cm2) corresponding to the electrodeposition of MnO2.

The third column further indicates the loading (g/m2) or amount of noble metal (NM) in the mixed oxide and of the MnO2 topcoating (TC) deposited per unit surface area of the titanium sheet substrate (g/m2).

The last three columns in Table 1 relating to electrolytic test data respectively indicate the corresponding anode test current density (ACD in A/m2w), anode potential (AP in V/NHE), and time or test duration in hours, marked with an asterisk when the test sample was still operating at that time, while the duration is underlined where the anode had failed.

It may be noted that the preparation of electrode samples Mel, C49 and B03 in Table 1 differed from that described above in that the titanium substrate of Mel was etched with HCI (instead of oxalic acid), while the MnO2 topcoating of B03 was heat treated at 330 C (instead of 400 C), and that of C49 at 400 C but in static air.

Electrode sample CHI was removed from the test cell after 1000 hours of stable operation at 7500 A/m2 and was then subjected to X-ray diffraction (XRD) analysis, which showed that about 75-80 % of the original p MnO2 coating still remained, without having undergone any notable structural changes.

TABLE 1 COATING LOADING ELECTROLYTIC TEST APPLN x SOLN NM REF. ELECTROPLATING TC ACD AP TIME h/A cm2 g/m2 A/m2 V/NHE h B 96 4 xS 6 0.5 Ir 500 1.64 20/1.5 320 Mn 03 1.76 8300 C 37 ,, 0.5 Ir 1.75 7900 " 400 Mn O2 1.76 B O3 4 X S 6 0.5 Ir 1.94 21/1.5 390 Mn O2 2500 fail 400 Me 2 4 x S 6 0.5 Ir 4500 1.94 20/1.5 381 Mn O2 fail 1300 SM1 ,, 0.5 Ir 1.86 418 Mn 02 fail 2000 Me 1 0.5 Ir ,, 1.87 424 Mn 03 fail 1300 D40 a ,, 0.5 Ir 1.95 " 493 Mn 02 fail 2000 SM 3 4 4XS6 5 6 0.5 Ir 2.0 3/10 360 Mn O2 fail 680 SM 2 4xS6 0.5 Ir 1.90 1.5/20 328 Mn O2 fail 1550 D 45 4 x S 6 0.5 Ir 7500 1.94 20/1.5 395Mn02 fail 785 SM 7 " 0.5 Ir 1.97 410 Mn 02 fail 913 C 49 ,, 0.5 Ir 1.94 " 260 Mn 03 fail 253 D 40 B 4 x S6 0.5 Ir 1.95 1.5/20 200 Mn 2 fail 420 CH 1 5 us 6 0.9 Ir 1000* 400 Mn O, EXAMPLE 2 Table 2 gives the reference B 65. Fl 2 and SM5 and corresponding data of three electrode samples which were pretreated and provided with a mixed oxide substrate in the manner described in Example 1. However. instead of electrodepositing the MnO2 topcoating, it was formed in this case, for purposes of comparison, by thermal decomposition of manganese nitrate applied in solution to the mixed oxide surface layer. Table 2 gives the corresponding data for all these samples in the same manner as in Table 1.

A comparison of the results given in Tables 1 and 2 shows that higher lifetimes under similar conditions were achieved with the electroplated manganese dioxide coatings of Example 1.

COATING LOADING ELECTROLYTIC TEST APPLN x SOLN NM REF. TC ACD AP TIME g/m2 A/m2 V/NHE h B65 4Xs6 0.5 Ir 500 1.86 nitrates 400 Mn O2 fail 6200 F12 4X s6 0.5 Ir 7500 2.02 nitrates 360 MnO2 fail 230 SM 5 4Xs6 0.5 ,, 1.96 nitrates 400 MnO:, fail 350

Claims (11)

1. A method of manufacturing a catalytic electrode comprising a manganese dioxide coating, a valve metal base, and an intermediate electro-conducting protective layer, characterized by the steps of: a) applying to the valve metal base surface at least one chloride of a noble metal selected from the group consisting of iridium, rhodium and ruthenium, and further comprising hydrochloric acid in a concentration which is selected so that the molar ratio of hydrogen chloride to said noble metal chloride present in said solution lies in a given range of molar ratios, b) slowly drying the solution thus applied so that the hydrochloric acid remains in contact with the underlying valve metal base surface for a sufficient time to convert a significant amount of valve metal at said surface to a corresponding chloride, in order to thereby provide at the valve metal base surface a chloride mixture comprising the corresponding valve metal chloride and said noble metal chloride in given proportions corresponding to said molar ratio, c) subjecting the resulting chloride mixture to heat treatment in an oxidizing atmosphere at a temperature lying in the range from 400'C to about 600'C so as to thereby convert said chloride mixture to an electro-conductive mixed oxide integrated in the valve metal base surface, d) repeating the sequence of steps a) to c) a number of times sufficient to gradually form a thin integrated oxide layer of desired thickness which consists essentially of said mixed oxide comprising said valve metal and said noble metal in given proportions corresponding to said molar ratio, e) forming on the resulting mixed oxide layer integrated in the valve metal base surface a catalytic coating of manganese dioxide.

2. The method of claim 1, characterized in that said catalytic coating is formed by electrodepositing manganese dioxide in an amount corresponding to at least 100 grams of said coating per square meter of the valve metal base surface area, and the electrodeposited manganese dioxide coating is subjected to heat treatment so as to improve the electrode performance.

3. The method of claim 1 or 2, characterized in that said chloride mixture is heat treated in a temperature range from about 450"C to about 520"C.

4. The method of claim 1, 2 or 3, characterized in that said molar ratio of the amounts of hydrogen chloride to the noble metal chloride respectively present in said solution is selected from the range between 1:1 and 100:1.

5. The method of claim 4, characterized in that said molar ratio is selected between 3:1 and 30:1

6. The method of any one of claims 1 to 5, characterized in that the molar concentration of noble metal chloride present in said solution is selected in the range between 1 X 10-2 and 25 X 10 2 mole per liter of solution.

7. The method of claim 6, characterized in that said molar concentration of noble metal chloride lies between 2 X 10 2 and 10 X 10-2 mole per liter.

8. The method of claim 7, characterized in that said noble metal chloride molar concentration corresponds to about 2.5 X 10-to about 7.5 x 10-2 mole per liter.

9. The method of any one of claims 1 to 8, characterized in that the molar concentration of HCI in said solution is selected between 14 x

10 7 and 3 mole per liter of solution.

1 0. The method of any one of claims 1 to 9, characterized in that said solution applied to the valve metal base surface comprises a non-aqueous solvent which slowly evaporates during the drying step (b) while leaving the hydrochloric acid in contact with said surface for a sufficient time to provide for conversion of the valve metal to the corresponding chloride.

11. The method of claim 10, characterized in that said solvent is an alcohol.

1 2. The method of claim 11, characterized in that said solvent is isopropylalcohol.