Electrochemical behavior of the bare indium-tin oxide (ITO) electrode in 1 M NaOH electrolyte has been studied in a wide range of current density and applied charge in view of its possible application as a counterelectrode in rare earth optical windows. Nature and kinetics of both cathodic and anodic processes developed on ITO depend on electrolyte composition as well as on transformations occurring with the electrode. Contribution of oxygen vacancies and ions O 2 - is crucial for the electrochemical behavior of ITO. At the cathodic polarization, the electrode components are deeply reduced so that ITO is gradually and irreversibly converted to a metallic mirror with a noticeable decrease of oxygen content. At high anodic current density, the ITO electrode undergoes modifications and its conductivity decreases probably also due to the change of oxygen content in the oxides lattice. A phenomenological description of all the processes involved in ITO electrochemical performance and their reversibility is proposed and discussed. A stable operation of ITO in electrochemical devices can be achieved only in the case of usage of external RedOx dissolved in the electrolyte or deposited on the ITO electrode. Such active components could be selected from the different systems studied for flat panel displays.

Electrochemistry of the Indium-Tin Oxide Electrode in 1 M NaOH Electrolyte

The behavior of a thin-film yttrium electrode (150-200 nm) capped by a palladium layer (20 nm) toward electrochemical hydrogen uptake was studied in I M NaOH electrolyte under galvanostatic conditions. The system is of interest for smart optical windows/ mirrors in devices with changeable transparency. The electrochemical behavior of the Y/Pd electrode depended on many factors, e.g., whether the loading is performed on a fresh or used electrode, current density, presence of reducible components in the electrolyte, time of spontaneous open-circuit potential relaxation and/or anodic discharge. Residual hydrogen kept by the system and its interaction with some electrolyte components is a reason for this behavior. A decisive role of the palladium layer in electrochemical kinetics was elucidated for the first time. We assume that the kinetics of the hydrogen uptake by underlying yttrium as well as the corresponding hydrogen release process is controlled by diffusion from/through hydrogenated palladium. The constant potential hydrogenation process realized at 0.045 V vs. the reversible hydrogen electrode (RHE) was proposed to be the α-β Pd hydride transition. Formation of the β-Y dihydride phase is completed on the way to the -0.095 V potential level (vs. RHE) and is irreversible. In contrast, the β-γ Y hydride transition realized at this potential level is reversible and could be repeated many times. The last potential drop to -0.345 V (vs. RHE) is a hydrogen evolution process and, simultaneously, a metal-semiconductor (and optical) transition in the γ-YH 3 phase. In a discharge process, a β-Y dihydride of different stoichiometry and α-Pd hydride are probably the only species that could be obtained. The kinetics depends extremely on the discharge conditions (open-circuit relaxation or anodic polarization), and the process shows two potential levels. Under anodic hydrogen release, surface Pd oxidation could also take place. We have proposed and discussed a scheme for hydrogenation/release in the Y/Pd electrode under different experimental conditions.

Kinetics of the Electrochemical Loading of Hydrogen into Thin Yttrium Films Covered by Palladium

Electrochemically induced switching in SmHx thin films

Publisher Summary This chapter summarizes various studies carried out on different generations of rare earth metal switchable mirrors based on polycrystalline, epitaxial layers, rare earth–Mg alloys, rare earth–Mg multilayers structures, and nanoparticles In case of polycrystalline and epitaxial rare earth metals, switching behavior measured in terms of optical and electrical contrast, switching and recovery times and transparency and color neutrality of the trihydride state is explained in terms of intrinsic effects such as crystal structure change, phase modification, lattice contraction/expansion, and lateral and longitudinal H diffusion during hydrogenation and dehydrogenation Moreover, a large scope for fabricating a variety of devices based on hydrogen induced structural, optical, and electrical changes and the possibility of new physics required to explain the intriguing phenomenon accompanying the switchable mirror effect – provided the necessary impetus for applied and basic research in this area More work needs to be carried out to understand R metal–palladium (Pd) interface Use of Pd nanoparticles has been shown to improve the switching properties and it may be possible to modify Pd layers by other means It may also be possible to use chemical methods for growing rare earth layer

Switchable metal hydride films

The optical switching properties of a solid state device based on Ho0.6Mm0.4Co2 (HMC) alloy thin film as a switching active layer, water pretreated Nafion membrane as a solid electrolyte, and a transparent conducting indium tin oxide (ITO) as a counterelectrode are investigated. The device is simple and has a reduced layer sequence of HMC/Pd/Nafion/ITO. The reversible optical switching of this device has been studied during electrochemical galvanostatic charging-discharging as well as cyclic voltammetric measurements. Further, the optical switching durability of the device has been tested by repeated electrochemical hydrogenation-dehydrogenation, and the variations in the optical switching properties are discussed. The special characteristic of the device is that it can reversibly switch between a metallic reflecting state and a semiconducting transparent state by a small reversible applied current/voltage indicating the potential substitution for conventional electrochromic devices.

Optical switching properties of RCo2-type alloy hydride based solid state device

A reversible optical transition in rare-earth metal hydrides can take place in a gas phase (when H 2 is injected into the chamber with rare-earth metal film and pumped out periodically) as well as in an electrolyte solution (when the electrode of active metal is polarised cathodically and depolarised). The liquid hydrogenation process, though more reliable from technological point of view, still presents some undesirable side effects and inferior longevity due to high reactivity of metals and their hydrides towards electrolyte solutions and enhanced corrosion. In this paper, we present our results on electrochemical behaviour of the Y/Pd cathode and its reversible optical switching paying attention to some common problems that might affect the device work and even cause its destruction. Based on these data, we have discussed a possible design and optimum operational conditions of the planar electrochemical cell: type of electrolyte used in the cell (liquid, solid or polymeric); role and design of the anodic electrode and the anodic process accomplished on it; sealing problems due to the necessity to obtain a hermetically closed cell with a liquid electrolyte inside and a possible gas evolution under the operation.

A. Sanchez Bolinchez

Papers

Design and operational conditions of small electrochemical cell for optical switching employing hydrogenation of Pd/Y structure