Electrometallurgy in aqueous media is largely based on the use of Pb and Pb alloy anodes, which exhibit reasonable stability and very low costs. However, because of the new and more strict environmental legislation, there is at present a tendency to reduce their use, due to their strong polluting character, and this prompts, in turn, the research on alternative anode materials. These materials have to fulfill the requirement of a long service life, together with a reasonable catalytic activity toward the oxygen evolution reaction (o.e.r.). Ordinary dimensionally stable anodes (DSA®), classically based on RuO2-containing mixed-oxide films, although exhibiting a quite good catalytic activity toward o.e.r., have a very poor service life, which excludes any interest for solutions based on them. At variance, electrode materials based on IrO2 seem more promising from the viewpoint of stability and maintain good catalytic properties. At this point, although a number of papers have been already published (e.g.: [1, 2]), the goal of the optimization of the properties of IrO2-based film electrodes has not been attained yet. Scope of the present work was the preparation and characterization of IrO2-SnO2 films deposited on Titanium, elucidating in particular the influence of the electrode film composition and noble-metal-oxide loading on catalytic activity toward the o.e.r. and service life. Accordingly, several electrode compositions were chosen between 1 and 70 mol% of IrO2, and the preparation was carried out by oxidative pyrolysis of precursor salt mixtures deposited onto Ti plate, making use of IrCl4 and SnCl4 precursor solutions, mixed in the appropriate ratios. The microstructural investigation, carried out by wide-angle X-ray scattering, has shown that a solid solution between the IrO2 and SnO2 rutile structures is formed across the whole phase diagram, and the Vegard’s law is followed. In all cases, an expansion of the cell volume was also observed, in agreement with observations made in other papers, concerning other oxides prepared by thermal methods or RF reactive sputtering [3]. Interestingly, the average crystallite size, estimated by the Scherrer method, was found to be quite small (of the order of 3-6 nm) and relatively unaffected by the film composition. These data support the expectation that the electrode materials, under such a dispersed morphology, should exhibit high charge storage capacity as well as high catalytic activity toward “difficult” reactions, like o.e.r. is. In fact, both expectations have been fully confirmed by experiments. Voltammetric charges may be as high as 0.15 C cm-2 and extended linearity regions with slopes of 43-46 mV in Tafel plots have been found, supporting the hypothesis that the rearrangements of the electroactive / catalytically active sites in the electrode films take place with low activation energies. These features are accompanied by a very long service life, under accelerated test conditions (solution: 2 N H2SO4, J=30,000 A m-2, T = 60 °C), which can attain values as high as ~107 Ah m-2, allowing the conclusion that electrode films of IrO2 stabilized with SnO2 are promising materials for industrial applications where stability and catalytic activity under o.e.r. are required.

NEW ELECTRODIC MATERIALS FOR ELECTROMETALLURGIC APPLICATIONS: CATALYTIC AND ENVIRONMENTAL ASPECTS

DE BATTISTI, Achille;FERRO, Sergio;URGEGHE, Christian;MOROZOV, Alexander
2004

Abstract

Electrometallurgy in aqueous media is largely based on the use of Pb and Pb alloy anodes, which exhibit reasonable stability and very low costs. However, because of the new and more strict environmental legislation, there is at present a tendency to reduce their use, due to their strong polluting character, and this prompts, in turn, the research on alternative anode materials. These materials have to fulfill the requirement of a long service life, together with a reasonable catalytic activity toward the oxygen evolution reaction (o.e.r.). Ordinary dimensionally stable anodes (DSA®), classically based on RuO2-containing mixed-oxide films, although exhibiting a quite good catalytic activity toward o.e.r., have a very poor service life, which excludes any interest for solutions based on them. At variance, electrode materials based on IrO2 seem more promising from the viewpoint of stability and maintain good catalytic properties. At this point, although a number of papers have been already published (e.g.: [1, 2]), the goal of the optimization of the properties of IrO2-based film electrodes has not been attained yet. Scope of the present work was the preparation and characterization of IrO2-SnO2 films deposited on Titanium, elucidating in particular the influence of the electrode film composition and noble-metal-oxide loading on catalytic activity toward the o.e.r. and service life. Accordingly, several electrode compositions were chosen between 1 and 70 mol% of IrO2, and the preparation was carried out by oxidative pyrolysis of precursor salt mixtures deposited onto Ti plate, making use of IrCl4 and SnCl4 precursor solutions, mixed in the appropriate ratios. The microstructural investigation, carried out by wide-angle X-ray scattering, has shown that a solid solution between the IrO2 and SnO2 rutile structures is formed across the whole phase diagram, and the Vegard’s law is followed. In all cases, an expansion of the cell volume was also observed, in agreement with observations made in other papers, concerning other oxides prepared by thermal methods or RF reactive sputtering [3]. Interestingly, the average crystallite size, estimated by the Scherrer method, was found to be quite small (of the order of 3-6 nm) and relatively unaffected by the film composition. These data support the expectation that the electrode materials, under such a dispersed morphology, should exhibit high charge storage capacity as well as high catalytic activity toward “difficult” reactions, like o.e.r. is. In fact, both expectations have been fully confirmed by experiments. Voltammetric charges may be as high as 0.15 C cm-2 and extended linearity regions with slopes of 43-46 mV in Tafel plots have been found, supporting the hypothesis that the rearrangements of the electroactive / catalytically active sites in the electrode films take place with low activation energies. These features are accompanied by a very long service life, under accelerated test conditions (solution: 2 N H2SO4, J=30,000 A m-2, T = 60 °C), which can attain values as high as ~107 Ah m-2, allowing the conclusion that electrode films of IrO2 stabilized with SnO2 are promising materials for industrial applications where stability and catalytic activity under o.e.r. are required.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/1685209
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