REVERSIBLE ELECTRODEPOSITION OF SILVER MIRROR SURFACES FROM AN IONIC LIQUID ONTO A TRANSPARENT CONDUCTOR

Abstract

Reversible electrochemical mirror (REM) systems, which reversibly electrodeposit a reflective metal layer onto a transparent substrate, are a promising technology with applications in light and thermal control. Low weight and intermediate tunability make REM panels a desirable replacement for existing satellite thermal control, but no current REMs use vacuum-stable solvents. The present research analyzes a silver REM system using 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM TFSI), a vacuum-stable ionic liquid, as an electrolyte. Cyclic voltammetry, chronoamperometry, scanning electron microscope (SEM) imaging and reflectometry were used to investigate the electrochemical deposition and stripping of silver onto indium tin oxide (ITO) transparent thin films from BMIM TFSI. These methods, along with UV-visible spectroscopy, energy-dispersive X-ray (EDX) spectroscopy and line profilometry, were also used to determine the influences of the redox additives copper(II) chloride (CuCl2), tetrabutylammonium bromide (TBA Br) and BMIM chloride on the system. The pure silver system was found to produce an initial high-quality mirror, but repeated cycling led to electrically disconnected silver deposits that could not be removed, interfering with subsequent depositions and producing unreflective surfaces. The addition of 10 mM CuCl2, 200 mM TBA Br and 252 mM BMIM Cl greatly improved the electrochemical and optical reversibility of a 50 mM solution of Ag TFSI in BMIM TFSI, plating 213 mirrors reversibly with reflectance values of up to 34% on the ITO side and 55% on the electrolyte side relative to an industrial-grade mirror. The additives are thought to bypass the failure mechanism of the pure system by 1) chemically oxidizing electrically-disconnected silver deposits and 2) providing more energetically favorable ligands to complex with Ag+.