Jonathan L. Slack

Switchable mirrors based on nickel-magnesium films

A new type of electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin,magnesium-rich Ni-Mg films prepared on glass substrates by cosputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on reduction in alkaline electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extraction,and to protect the metal surface against oxidation.

Mixed metal films with switchable optical properties

Thin, Pd-capped metallic films containing magnesium and first row transition metals (Mn, Fe, Co) switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous hydrogen or following cathodic polarization in an alkaline electrolyte. Reversion to the reflecting state is achieved by exposure to air or by anodic polarization. The films were prepared by co-sputtering from one magnesium target and one manganese, iron, or cobalt target. Both the dynamic optical switching range and the speed of the transition depend on the magnesium-transition metal ratio. Infrared spectra of films in the transparent, hydrided (deuterided) states support the presence of the intermetallic hydride phases Mg3MnH7, Mg2FeH6, and Mg2CoH5.

Electrochromism in copper oxide thin films

Transparent thin films of copper(I) oxide prepared on conductive SnO2:F glass substrates by anodic oxidation of sputtered copper films or by direct electrodeposition of Cu2O transformed reversibly to opaque metallic copper films when reduced in alkaline electrolyte. In addition, the same Cu2O films transform reversibly to black copper(II) oxide when cycled at more anodic potentials. Copper oxide-to-copper switching covered a large dynamic range, from 85 percent and 10 percent photopic transmittance, with a coloration efficiency of about 32 cm2/C. Gradual deterioration of the switching range occurred over 20 to 100 cycles. This is tentatively ascribed to coarsening of the film and contact degradation caused by the 65 percent volume change on conversion of Cu to Cu2O. Switching between the two copper oxides (which have similar volumes) was more stable and more efficient (CE = 60 cm2/C), but covered a smaller transmittance range (60 percent to 44 percent T). Due to their large electrochemical storage capacity and tolerance for alkaline electrolytes, these cathodically coloring films may be useful as counter electrodes for anodically coloring electrode films such as nickel oxide or metal hydrides.

Electrochromic lithium nickel oxide thin films by RF-sputtering from a LiNiO2 target

Thin films of lithium nickel oxide were deposited by rf sputtering from a stoichiometric LiNiO 2 target. The films exhibit excellent reversibility in the potential range 1.1 to 3.8 V versus Li/Li + and could be cycled in a liquid electrolyte half cell with a switching range ΔT Photopic close to 70%. The photopic coloration efficiency of this anodically coloring material was typically -30 to -40 cm 2 C -1 . The switching performance of a device utilizing a lithium nickel oxide film as counter electrode and a tungsten oxide electrochromic film is reported. The device switched from 75% photopic transmittance to 2%, with color neutrality and a coloration efficiency of 65 cm 2 C -1 .

Effect of hydrogen insertion on the optical properties of Pd-coated magnesium lanthanides

LBNL-42277 OM-395 Presented at the Third International Meeting on Electrochromics in London, England, September 8, 1998 and accepted for publication in Electrochimica Acta. Effect of Hydrogen Insertion on the Optical Properties of PD-Coated Magnesium Lanthanides K. von Rottkay, M. Rubin, F. Michalak, R. Armitage, T. Richardson, J. Slack, *P.A. Duine Windows and Daylighting Group Building Technologies Department Environmental Energy Technologies Division Ernest Orlando Lawrence Berkeley National Laboratory University of California Berkeley, CA 94720 *Philips Research Laboratories Prof. Holstlaan 4 5656 AA Eindhoven The Netherlands September 1998 This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, State and Community Programs, Office of Building Systems of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

X-Ray absorption spectroscopy of transition metal–magnesium hydride thin films

Mixed metal thin films containing magnesium and a first-row transition element exhibit very large changes in both reflectance and transmittance on exposure to hydrogen gas. Changes in electronic structure and coordination of the magnesium and transition metal atoms during hydrogen absorption were studied using dynamic in situ transmission mode X-ray absorption spectroscopy. Mg K-edge and Ni, Co, and Ti L-edge spectra reflect both reversible and irreversible changes in the metal environments. A significant shift in the nickel L absorption edge shows it to be an active participant in hydride formation. The effect on cobalt and titanium is much less dramatic, suggesting that these metals act primarily as catalysts for formation of magnesium hydride.

Electroanalytical study of the viability of conversion reactions as energy storage mechanisms

Storing electrochemical energy by means of fully reducing transition metal compounds to their metallic state is attractive due to the large amounts of charge that are produced. However, while more reversible than originally envisioned, electrochemical conversion reactions are accompanied by large inefficiencies. These inefficiencies are associated with hysteresis between the potentials of reduction and re-oxidation, as well as the loss of capacity when reversing back to the initial state. This work presents a series of results collected from different kinds of electroanalytical experiments. The use of data from conventional powder electrodes as well as thin films offered the opportunity to compare measurements of common occurrence in the literature with more sophisticated experiments carried out at higher temperature, which provided a complete picture of the kinetic nature of the processes involved. This picture is supportive of the thermodynamic nature of potential hysteresis, and indicates that coulombic inefficiencies stem from the low reversibility of the conversion reaction. Taken together, the results cast a clear light on the significant challenges that lie ahead if this kind of reactivity is to become technologically relevant in devices such as Li-ion batteries.

Analysis of Durability in Lithium Nickel Oxide Electrochromic Materials and Devices

Thin films of lithium nickel oxide were deposited by sputtering and laser ablation from targets of pressed nickel oxide and lithium oxide powders. These films were assembled into electrochromic test devices with tungsten oxide as the opposite electrode and a polymer electrolyte. Analysis of the failure modes was carried out at several levels: The composition and structure of the films were examined before and after cycling using a variety of techniques, such as infrared spectroscopy, nuclear-reaction analysis, Rutherford backscattering spectrometry, X-ray diffraction and atomic force microscopy. Absorption of water vapor was found to be a major factor determining the cyclic stability of the films. A new technique is described for incorporating reference electrodes made from an electronically isolated corner into devices. This structure enabled identification of potential problems associated with a particular interface. Finally, some of the devices were disassembled and the components examined. For example, a small quantity of the polymer was extracted and studied by gas chromatography and mass spectroscopy. Small organic fragments were discovered which correspond to expected weak points in the polymer structures.

Size and composition-controlled fabrication of thermochromic metal oxide nanocrystals

Finding new methods for the fabrication of metal oxide nanocrystals with high control on their composition, size and crystallinity is paramount for making large-area and low-cost optical coatings. Here, we demonstrate the fabrication of thermochromic VO2 nanocrystals using a physical vapour deposition-based route, with high control over their composition, size and crystallinity. This technique presents great potential to be scaled up and integrated with in-line coaters, commonly used for large-area deposition. Optimum crystallization of the VO2 nanoparticles is achieved after post-growth annealing at 350 ◦ C, a temperature drastically lower than that required by chemical or implantation fabrication methods. The obtained nanoparticle thin films exhibit superior modulation of the transmittance in the visible and near IR portion of the spectrum as compared to conventional VO2 thin films due to plasmonic effects, opening up a new horizon in applications such as smarts windows.

Effective medium approximation of the optical properties of electrochromic cerium-titanium oxide compounds

Cerium titanium oxide samples produced by sol-gel have been compared against sputtered and pulsed laser deposited films over a wide range of different compositions. X-ray diffraction was used to investigate the structural properties of the compound material existing in a two-phase mixture MAO2-MBO2. The optical properties were evaluated over the whole solar spectrum by variable angle spectroscopic ellipsometry combined with spectrophotometry. The spectral complex refractive index was determined for CeO2 and TiO2, as well as for their compounds. To reduce the large number of permutations in composition of multi-component oxides it would be useful to be able to predict the properties of the mixtures from the pure oxide components. Therefore these results were compared to those obtained by effective medium theory utilizing the optical constants of CeO2 and TiO2. In order to investigate the performance as passive counter-electrode in Li+ based electrochromic devices the films were tested by cyclic voltammetry with in-situ transmission control. Chemical composition was measured by Rutherford backscattering spectrometry. Surface morphology was analyzed by atomic force microscopy.