Optical Switching of Y‐Hydride Thin Film Electrodes: A Remarkable Electrochromic Phenomenon

The optical appearance of yttrium thin film electrodes can be electrochemically switched from mirrorlike to highly transparent by making use of the hydride‐forming properties of Y. A strong alkaline solution is a suitable electrolytic environment for obtaining stable electrochromic electrodes. The presence of a thin Pd layer covering the Y electrode is essential in providing a sufficiently high electrocatalytic activity for the electrochemical charge‐transfer reaction. Hydrogen is irreversibly bound in Y‐dihydride and reversibly bound in Y‐trihydride. The optical changes are reversible and are induced within a narrow hydrogen concentration range, making this typical electrode material interesting for application in a new type of electrochromic devices.