Reversible Electrochemical Mirror Devices Using Space Compliant Ionic Liquid Electrolytes
Authors: Garich Holly , Liu Danny X. , Inman Maria , Taylor E. J. , Peng Thomas , Davis James H. , O'Brien Richard , Tench D. Morgan 
Published: February 2020

Reversible electrochemical mirror devices function through reversible redox reactions that alternate between deposition of a highly reflective thin metallic film (hence the term mirror) and complete oxidation of the metallic film during the erasure cycle. These devices generally utilize transparent, conductive substrates such as those based on indium tin oxide type films applied to glass or plastic transparent substrates, though other substrate materials could be used in the build of these devices. REM devices may be built to either facilitate reflection and transmission or reflection and absorption of radiation sources depending on the nature of the counter electrode used. REM devices may be used for various applications including auto-dimming mirrors for the automotive industry, electrochromic windows for the aviation industry, smart glass for the architectural industry and mirrors or thermal emitters for deployment on orbital platforms.

Conventional REM devices utilize electrolytes based on organic solvents such as gamma-butylrolactone and dimethyl sulfoxide. For space based applications, these electrolytes are unsuitable due to their vapor pressures and potential for evaporation if the cell seal is compromised. Room temperature ionic liquid electrolytes (RTIL) are an attractive alternative to these conventional systems due to their negligible vapor pressure in addition to their excellent chemical and thermal stability and their large electrochemical windows. RTIL base electrolytes may be tailored to deliver specific properties based on their cation and anion species, making the electrolytes tunable for specific applications. RTILs with anionic species such as chloride, bromide and iodide have been evaluated as potential alternatives to the conventional organic electrolytes for REM devices. These systems have shown promise in terms of mirror formation and long cycle life; however, they exhibit high sensitivity to water, including atmospheric moisture, which can limit their usefulness in REM devices. The present work focuses on deposition and stripping of silver thin films from RTIL with relative low moisture sensitivity. Simple, two electrode cells were built using transparent electrodes with electrically conductive films (i.e. indium tin oxide, platinum and silver) and air and moisture stable RTIL electrolytes for electrochemical characterization and plating/stripping cycling experiments.

Highly reflective and reproducible silver mirror formation using air and moisture stable RTIL based electrolytes has been demonstrated. In the current work, experiments are being conducted to explore how the cell configuration and cell components influence deposition and oxidation processes as well as device cycling lifetime. Use of pulsed deposition and erasure steps have also been explored in simple REM systems containing RTIL electrolytes. Pulse and pulse reverse processes have the ability to improve the deposition process of reversible silver systems as compared to operation under constant voltage or constant current electric fields, namely the ability to enhance mass transport properties of the system as well as driving nucleation of the silver thin films over grain growth, which is anticipated to help in the stripping of the film from the substrate. Operation under pulse/pulse reverse electric fields is expected to achieve highly reflective metallic mirror films while maximizing the cycling lifetimes and preventing operational degradation of the electrolyte and cell components. This work outlines how cell configuration and device operating conditions influence deposition and erasure reactions as well as device cycle lifetime.

Acknowledgement: Funding for this work is gratefully acknowledged from Air Force STTR Contract Number FA9453-17-C-0490. Platinum coated ITO electrodes were prepared by Dr. John Bryan Plumley.