ELECTROCHROMISM IN ANODIC IRIDIUM OXIDE-FILMS .2. PH EFFECTS ON CORROSION STABILITY AND THE MECHANISM OF COLORATION AND BLEACHING

The oxidation state of iridium ions in an oxide film, grown electrochemically on an Ir metal reflector electrode, can be rapidly and reversibly modulated according to the electrochromic redox reaction by suitable choice of electrolyte composition and potential limits, the coloration and bleaching processes can be enected without causing oxide film growth or dissolution, or electrolyte decomposition. Color-bleach (c-b) cycles exhibit reflectance contrast changes, R/R, and charging times, t, suitable for electro-optic display devices, e.g. for a film 320 nm thick, R/R = 70% at ε = 546 nm and ≤ 80 msec. Long term corrosion stability is obtainable in mildly acidic sulfate electrolytes, e.g. 0.5M Na2SO4 at pH 3.5. Sulfate electrolytes are also apparently unique in preventing degradation of c-b response times owing to changes in film structure. Fast write-erase times are made possible by the highly porous and hydrated nature of the oxide film and the ready availability of the protons required for bleaching from an ‘internal’ source-free H2O molecules in the electrolyte contained within the film pores and/or bound H2O or OH groups on the oxide surface. The kinetics and mechanism of the electrochromic reaction in anodic iridium oxide films are discussed with particular reference to the requirement for preserving electroneutrality within the bulk oxide. Recent claims that this redox reaction can occur without exchange of protons with the electrolyte are shown not to be substantiated. Attractive features of the electrochromic iridium oxide system for display devices include: (i) broad spectral absorption; (ii) good contrast ratio; (iii) fast response; (iv) good corrosion stability; (v) good open-circuit memoir in the presence of water and dissolved 02; (vi) suitable threshold and switching voltage levels; and (vii) the ability to grow and reform the oxide layer in situ in the electro-optic display cell. The charge (~20 mC cm-2) and energy (~30 mJ cm-2) required for coloration and bleaching are similar to those required for other electrochromic oxide systems, e.g. the tungsten bronzes. © 1979, The Electrochemical Society, Inc. All rights reserved.