Iridium Anodic Oxidation to Ir(III) and Ir(IV) Hydrous Oxides

Abstract

Iridium oxide films (IROFs) are known to have an enhanced or the so-called super-Nernstian (< 59 mV/pH) pHsensitivity. The intention in the present study was to find out the reasons of such behavior and also to elucidate the nature of iridium anodic oxidation processes. The methods employed were combined cyclic voltammetry and chronopotentiometry. Iridium layers of 0.1 to 0.2 mm thickness, deposited thermally on titanium or gold-plated titanium substrates, were used for investigations. IROFs on the surface of working electrodes were formed anodically by applying a constant potential in deaerated and oxygen-containing solutions of 0.5 M H 2 SO 4 , 0.1 M KOH and 0.5 M H 3 PO 4 þ KOH. Linear pH-dependences of the stationary open-circuit potential with the slopes close to 59 mV/pH were found for iridium electrode oxidized at 0.4 V -0.8 V (RHE) in deaerated and at 0.8 V -1.2 V (RHE) in O 2 -containing solutions. They were attributed to reversible Ir/Ir(OH) 3 and Ir/ IrO 2 · nH 2 O metal-oxide electrodes, respectively. It has been suggested that the main current peaks seen in the voltammograms of iridium electrode in acid and alkaline solutions are of different nature. The difference between iridium electrode surface states in acid and alkaline solutions has been presumed to be the main reason of super-Nernstian pH-sensitivity of the IROFs. On the basis of the results obtained standard potential of Ir/Ir(OH) 3 electrode and the solubility product of Ir(OH) 3 have been evaluated: E Ir=IrðOHÞ 3 ¼ 0.78 AE 0.02 V and K sp ¼ 3.3 Â 10 À64 .

Oxygen evolution activity and stability of iridium in acidic media. Part 2. – Electrochemically grown hydrous iridium oxide

“…Hence, taking into account that the only simple aquo-ions formed by iridium are [Ir(H 2 O) 6 ] 3+ , thus iridium is in oxidation state Ir(III) [66] and well known instability of Ir(III) complexes such as Ir(OH) 3 or Ir 2 O 3 which are readily oxidized to IrO 2 in presence of oxygen [35,66] or, as for Ir 2 O 3 , have relatively high solubility at low pH [67], we hypothesize that precursors for dissolution and dissolved ions are oxygenated Ir(III) species but not Ir(VI) as suggested earlier.…”

“…This model explains why hydrous oxide forms predominantly during application of a potential program consisting of consequent anodic and cathodic perturbations but not when constant anodic potential is used. Although, it should be noted, that the formation of hydrous oxide at a moderate constant potential was reported in literature [34,35]. There is still, however, no consensus on some aspects of electrochemistry of such oxides, e.g.…”

“…There is still, however, no consensus on some aspects of electrochemistry of such oxides, e.g. oxidation/reduction behavior in acidic and alkaline electrolytes [35][36][37].…”

Screening Bifunctional Pt Based NSTF Catalysts for Durability with the Rotating Disk Electrode: The Effect of Ir and Ru

“…There could be other factors that contribute to the overall ORR activity of anodically oxidized Ir on Pt. For instance the hydrated 47 and porous 48 nature of anodically formed IrO 2 may be able to permit some oxygen reduction on the underlying Pt.…”

Metal Oxides and Hydroxides as Rechargeable Materials for Photocatalysts with Oxidative Energy Storage Abilities

“…Ru(III)O x (OH) 3¹2x ). 28 The redox waves in CV in 0.1 M aqueous NaNO 3 of the Ir oxide at around +1.0 V and Ru oxide at around +0.9 V were assigned to Ir(III/IV) 31,32 and Ru(III/IV), 33 Electrochemistry, 82(9), 749-751 (2014) and discharged at 500 nA cm ¹2 [ Fig. 3(b)].…”