Cation insertion reactions of electrochromic tungsten and iridium oxide films

Abstract

Electrochemical coloration mechanisms of electrochromic (EC), amorphous tungsten and iridium oxide films have been investigated by direct elemental analysis of their compositions in oxidized and reduced states. Combined Rutherford backscattering and nuclear-reaction-analysis techniques were employed to determine the identities and concentrations of ions injected as charge compensators from the contacting electrolyte during EC redox reactions. In aqueous electrolytes, the cathodic coloration of an anodically formed, hydrous tungsten oxide film (composition $\ensuremath{\sim}\mathrm{W}{\mathrm{O}}_{3}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$), is accompanied by injection of protons, in accordance with mechanisms determined for anhydrous W${\mathrm{O}}_{3}$. The anodic coloration and coloration bleaching of an anodically formed, hydrous iridium oxide (IROX) film, which has a varying degree of hydration throughout its thickness, is accompanied by ejection and injection of ${\mathrm{H}}^{+}$ and ${\mathrm{H}}_{3}$${\mathrm{O}}^{+}$ ions. In nonaqueous aprotic electrolytes, the slow but reversible insertion of ${\mathrm{Li}}^{+}$ ($r=0.60$ \AA{}) and ${\mathrm{Na}}^{+}$ ($r=0.95$ \AA{}) ions into IROX films has been successfully demonstrated for the first time. ${\mathrm{Li}}^{+}$ ions were found to exchange with ${\mathrm{H}}^{+}$ ions in these hydrous films. No evidence could be found for insertion of ${\mathrm{K}}^{+}$ ($r=1.33$ \AA{}), ${\mathrm{F}}^{\ensuremath{-}}$ ($r=1.36$ \AA{}), or O${\mathrm{H}}^{\ensuremath{-}}$ ($r=1.55$ \AA{}) ions as charge compensators. These results provide a measure of the size and concentration of vacancies and defects in amorphous IROX films.