Electrocatalysis induced by surface redox activities on conductive metal oxide electrodes

Abstract

Iridium dioxide electrodes form part of the dimensionally stable anodes (DSA®) and this electrode material is widely used in many industrial processes namely water electrolysis, metal electro-winning, cathodic protection and electro-organic synthesis due to the high electrochemical activity and stability of this electrode material. IrO2-based electrodes can be prepared using different techniques but the most common is the thermal decomposition of H2IrCl6 precursor solution on an inert substrate like titanium. Within the water stability potential domain, the charging/discharging process is attributed to the slow diffusion of protons within the IrO2 coating together with the electrical double layer capacitance. In fact the valence state of the Ir surface atoms of the coating varies from +IV to +VI in the potential domain between the on-set potentials of H2 and O2 evolution. Concerning the oxygen evolution reaction, direct evidence was found that the IrO2 coating participates actively in the reaction using an electrolyte solution containing isotopically labeled H218O. In fact, measurements of the relative amounts of electrogenerated 16O2 and 18O16O have demonstrated that the hydroxyl radicals coming from water discharge interact strongly with IrO2 resulting in the formation of the higher oxide (IrO3) and the decomposition of that oxide produces oxygen. IrO3 is thus the intermediate involved in the OER on these electrodes. During the oxidation of organic compounds, direct evidence was found by marking the IrO2 electrode with 18O that the higher valence state oxide IrO3 participates effectively also in this process. In fact, the oxidation of a solution of formic acid on a marked IrO2 coating containing 18O has shown that C16O18O is evolved proving that the oxidation of organic compounds occurs on IrO3 with competing side-reaction of oxygen evolution. The low overpotential of the OER allows performing selective electro-oxidation of a wide variety of organic compounds. In fact, the competing side reaction of oxygen evolution 'buffers' the potential around values where the oxidation products are not further oxidized showing that the IrO2 electrode is particularly suited for electro-organic synthesis. However, the current efficiency of the oxidation process remains low due to the competing side reaction of oxygen evolution.