Reversible Electrochemical Mirror Devices Using Room Temperature Ionic Liquid Electrolyte
Authors: Garich Holly , Hall Timothy D , Inman Maria , Taylor E. J. , Peng Thomas , Davis James , O'Brien Richard , Tench D. Morgan 
Published: February 2020

Devices that utilize reversible electrochemical mirrors (REM) have a number of applications including thermal and light management for terrestrial and space based systems. These systems use the concept of reversible electrodeposition to plate and strip highly reflective metallic films, i.e. mirrors, in the device to facilitate reflection and transmission or absorption of radiation. Figure 1 provides an illustration of a REM device whereby a transparent support, typically glass or plastic contains a transparent conducting layer that functions as the mirror electrode. An electrolyte containing the metal ions available for reduction and oxidation of the optically tailored metallic film is contained between the mirror electrode and the counter electrode, which may or may not be transmissive depending on the technology application. Electrodeposition of species such as silver, copper, tin, aluminum or gold generates a mirror like surface through the transparent electrode that facilitates reflection of radiation that aids in thermal management for satellite applications or light management for smart window applications. The mirror deposit can then be oxidized from the transparent electrode by applying a less negative or positive voltage with respect to the deposition voltage, permitting transmission of radiation. The cell may also be designed such as to cycle between reflective and absorptive states by changing the nature of the counter electrode.
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. Limited cycle lifetime in terms of the number of plating and stripping cycles with acceptable mirror formation was initially encountered. The thin conductive films applied to the transparent electrodes are susceptible to redox reactions which can limit their lifetimes, and as such relatively low voltages are applied in attempts to maintain the integrity of the electrodes and thus the REM device. Furthermore, the primary challenge with extended cycling of REM devices is the incomplete removal of the reflective surface leading to degradation of the mirror’s reflectivity with cycling. It is speculated that residual surface oxide effectively blocks complete stripping of the silver deposit and/or deposition of an acceptable reflecting mirror, adversely affecting device cycling lifetime. Therefore, a major challenge in the development of REM devices, whereby a reflective surface such as silver is alternatively plated and dissolved on a transparent conductive substrate, is that a buildup of undissolved reflective material on the electrode surface over successive cycles causes a loss of reflectivity and transmission. To counter this, an electrode maintenance cycle was introduced periodically within mirror plating and stripping cycles to aid in complete oxide removal, preparing the electrode surface for the next plating cycle. This maintenance cycle is introduced as a multiple cyclic voltammetry scan (cyclic potential sweep cleaning) and was found to extend device cycling lifetime. Data is presented on the development of conditions for plating and stripping cycles as well as the electrode maintenance cycle and scale up considerations and activities are also discussed.
Acknowledgement:
Funding for this work is gratefully acknowledged from Air Force STTR Grant Number FA9453-17-C-0490. Platinum coated ITO electrodes were prepared by Dr. John Bryan Plumley.

Figure 1