Electrode for oxygen evolution in industrial electrochemical processes

Abstract

The invention relates to an electrode for electrolytic processes, in particular to an anode suitable for oxygen evolution comprising a valve metal substrate, a catalytic layer, a protection layer consisting of oxides of valve metals interposed between the substrate and the catalytic layer and an outer coating of oxides of valve metals. The electrode of the invention is particularly suitable for processes of cathodic electrodeposition of chromium from an aqueous solution containing Cr (III).

Claims

1. Electrode suitable for oxygen evolution in electrolytic processes comprising a valve metal substrate, a catalytic layer, a protective layer consisting of valve metal oxides interposed between said substrate and said catalytic layer, and an external layer of a valve metal oxide, the valve metal oxide comprising tantalum pentoxide, said catalytic layer comprising mixed oxides of iridium, of tin and doping element M being bismuth, the molar ratio Ir:(Ir+Sn) ranging from 0.25 to 0.55 and the molar ratio M:(Ir+Sn+M) ranging from 0.02 to 0.15, wherein the specific loading of said valve metal oxides in said external layer ranges from 12 to 15 g/m.sup.2 referred to the oxides.

2. The electrode according to claim 1 wherein said molar ratio M:(Ir+Sn+M) ranges from 0.05 to 0.12.

3. The electrode according to claim 1 wherein said molar ratio Ir:(Ir+Sn) ranges from 0.40 to 0.50.

4. The electrode according to claim 1 wherein said oxides of iridium, of tin and doping element M in said catalytic layer consist of crystallites of average size below 5 nm.

Description

EXAMPLE 1

(1) A titanium sheet grade 1 of 200 mm×200 mm×3 mm size was degreased with acetone in a ultrasonic bath for 10 minutes and subjected first to sandblasting with corundum grit until obtaining a value of superficial roughness Rz of 40 to 45 μm, then to annealing for 2 hours at 570° C., then to an etching in 27% by weight H.sub.2SO.sub.4 at a temperature of 85° C. for 105 minutes, checking that the resulting weight loss was comprised between 180 and 250 g/m.sup.2.

(2) After drying, a protective layer based on titanium and tantalum oxides at a 80:20 molar ratio was applied to the sheet, with an overall loading of 0.6 g/m.sup.2 referred to the metals (equivalent to 0.87 g/m.sup.2 referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution obtained by addition of an aqueous TaCl.sub.5 solution, acidified with HCl, to an aqueous solution of TiCl.sub.4 and subsequent thermal decomposition at 515° C.

(3) A 1.65 M solution of Sn hydroxyacetochloride complex (hereinafter: SnHAC) was prepared according to the procedure disclosed in WO 2005/014885.

(4) A 0.9 M solution of Ir hydroxyacetochloride complex (hereinafter: IrHAC) was prepared by dissolving IrCl.sub.3 in 10% vol. aqueous acetic acid, evaporating the solvent, adding 10% aqueous acetic acid with subsequent solvent evaporation twice more, finally dissolving the product in 10% aqueous acetic acid again to obtain the specified concentration.

(5) A precursor solution containing 50 g/l of bismuth was prepared by cold dissolution of 7.54 g of BiCl.sub.3 under stirring in a beaker containing 60 ml of 10% wt. HCl. Upon completion of the dissolution, once a clear solution was obtained, the volume was brought to 100 ml with 10% wt. HCl.

(6) 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added to a second beaker kept under stirring. The stirring was protracted for 5 more minutes. 10 ml of 10% wt. acetic acid were then added.

(7) Part of the solution was applied by brushing in 7 coats to the previously treated titanium sheet, carrying out a drying step at 60° C. for 15 minutes after each coat and a subsequent decomposition at high temperature for 15 minutes. The high temperature decomposition step was carried out at 480° C. after the first coat, at 500° C. after the second coat, at 520° C. after the subsequent coats.

(8) In this way, a catalytic layer having an Ir:Sn:Bi molar ratio of 33:61:6 and a specific Ir loading of about 10 g/m.sup.2 was applied.

(9) The application of the external layer was then carried out (for an amount of 12 g/m.sup.2 referred to the oxides) by brushing in 8 coats of an aqueous TaCl.sub.5 solution, acidified with HCl. Three samples of 1 cm.sup.2 area were cut out from the electrode thus obtained and subjected to an accelerated duration test under anodic oxygen evolution, by measuring the deactivation time (defined as the time of operation required for observing a 1 V potential increase) in H.sub.2SO.sub.4 at 150 g/l, at a temperature of 60° C. and at a current density of 30 kA/m.sup.2. The average deactivation time of the three samples was found to be 600 hours.

(10) An anodic potential of 1.556 V/NHE was measured at 1000 A/m.sup.2.

EXAMPLE 2

(11) A titanium sheet grade 1 of 200 mm×200 mm×3 mm size was pre-treated and provided with a protective layer based on titanium and tantalum oxides at a 80:20 molar ratio as in the previous example. A precursor solution containing 50 g/l of tantalum was prepared by placing 10 g of TaCl.sub.5 in a beaker containing 60 ml of 37% by weight HCl bringing the whole mixture to boiling for 15 minutes under stirring. 50 ml of demineralised H.sub.2O were then added and the solution was kept under heating for about 2 hours until the volume was back to 50±3 ml. 60 ml of 37% by weight HCl were then added obtaining a clear solution, again brought to boiling until the volume was back to 50±3 ml. The volume was then brought to 100 ml with demineralised H.sub.2O. To a second beaker kept under stirring, 10.15 ml of the 1.65 M SnHAC solution of the previous example, 10 ml of the 0.9 M IrHAC solution of the previous example and 7.44 ml of the 50 g/l Ta solution were added. The stirring was protracted for 5 minutes. 10 ml of 10% by weight acetic acid were then added. Part of the solution was applied by brushing in 8 coats to the previously treated titanium sheet, carrying out a drying step at 60° C. for 15 minutes after each coat and a subsequent decomposition at high temperature for 15 minutes. The high temperature decomposition step was carried out at 480° C. after the first coat, at 500° C. after the second coat, at 520° C. after the subsequent coats.

(12) In this way, a catalytic layer having an Ir:Sn:Ta molar ratio of 32.5:60:7.5 and a specific Ir loading of about 10 g/m.sup.2 was applied.

(13) The application of the external layer was then carried out (for an amount of 15 g/m.sup.2 referred to the oxides) by brushing in 10 coats of an aqueous TaCl.sub.5 solution, acidified with HCl. Three samples of 1 cm.sup.2 area were cut out from the electrode thus obtained and subjected to an accelerated duration test under anodic oxygen evolution, by measuring the deactivation time (defined as the time of operation required for observing a 1 V potential increase) in H.sub.2SO.sub.4 at 150 g/l, at a temperature of 60° C. and at a current density of 30 kA/m.sup.2. The average deactivation time of the three samples was found to be 520 hours.

(14) An anodic potential of 1.579 V/NHE was measured at 1000 A/m.sup.2.

COUNTEREXAMPLE 1

(15) A titanium sheet grade 1 of 200 mm×200 mm×3 mm size was degreased and subjected first to sandblasting with corundum grit until obtaining a value of superficial roughness Rz of 70 to 100 μm, then to an etching in 20% by weight HCl at a temperature of 90-100° C. for 20 minutes.

(16) After drying, a protective layer based on titanium and tantalum oxides at a 80:20 molar ratio was applied to the sheet, with an overall loading of 0.6 g/m.sup.2 referred to the metals (equivalent to 0.87 g/m.sup.2 referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution—obtained by addition of an aqueous TaCl.sub.5 solution, acidified with HCl, to an aqueous solution of TiCl.sub.4—and subsequent thermal decomposition at 500° C.

(17) On the protective layer, a catalytic coating based on oxides of iridium and tantalum at a 65:35 weight ratio (equivalent to a molar ratio of about 66.3:36.7) was then applied, with an overall iridium loading of 10 g/m.sup.2. The electrode was heat-treated at 515° C. for 2 h, then the application of the external layer was carried out (for an amount of 15 g/m.sup.2 referred to the oxides) by brushing in 10 coats of an aqueous TaCl.sub.5 solution, acidified with HCl. Three samples of 1 cm.sup.2 area were cut out from the electrode thus obtained and subjected to an accelerated duration test under anodic oxygen evolution, by measuring the deactivation time (defined as the time of operation required for observing a 1 V potential increase) in H.sub.2SO.sub.4 at 150 g/l, at a temperature of 60° C. and at a current density of 30 kA/m.sup.2. The average deactivation time of the three samples was found to be 525 hours.

(18) An anodic potential of 1.601 V/NHE was measured at 1000 A/m.sup.2.

COUNTEREXAMPLE 2

(19) A titanium sheet grade 1 of 200 mm×200 mm×3 mm size was degreased and subjected first to sandblasting with corundum grit until obtaining a value of superficial roughness Rz of 70 to 100 μm, then to an etching in 20% by weight HCl at a temperature of 90-100° C. for 20 minutes.

(20) After drying, a protective layer based on titanium and tantalum oxides at a 80:20 molar ratio was applied to the sheet, with an overall loading of 0.6 g/m.sup.2 referred to the metals (equivalent to 0.87 g/m.sup.2 referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution obtained by addition of an aqueous TaCl.sub.5 solution, acidified with HCl, to an aqueous solution of TiCl.sub.4 —and subsequent thermal decomposition at 500° C.

(21) On the protective layer, a catalytic coating consisting of two distinct layers was then applied: a first layer (internal) based on oxides of iridium and tantalum at a 65:35 weight ratio (equivalent to a molar ratio of about 66.3:36.7), with an overall iridium loading of 2 g/m.sup.2 and a second layer (external) based on oxides of iridium, tantalum and titanium at a 78:20:2 weight ratio (corresponding to a molar ratio of about 80.1:19.4:0.5), for an overall iridium loading of 10 g/m.sup.2.

(22) The application of the external layer was then carried out (for an amount of 15 g/m.sup.2 referred to the oxides) by brushing in 10 coats of an aqueous TaCl.sub.5 solution, acidified with HCl. Three samples of 1 cm.sup.2 area were cut out from the electrode thus obtained and subjected to an accelerated duration test under anodic oxygen evolution, by measuring the deactivation time (defined as the time of operation required for observing a 1 V potential increase) in H.sub.2SO.sub.4 at 150 g/l, at a temperature of 60° C. and at a current density of 30 kA/m.sup.2. The average deactivation time of the three samples was found to be 580 hours.

(23) An anodic potential of 1.602 V/NHE was measured at 1000 A/m.sup.2.

(24) The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

(25) Throughout the description and claims of the present application, the term “comprise” and variations thereof such as “comprising” and “comprises” are not intended to exclude the presence of other elements, components or additional process steps.

(26) The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.