Reversible deposition of silver thin films has numerous industrial 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. Room temperature ionic liquids possess a number of unique properties that make them attractive for reversible silver systems, such as their relatively high conductivity, large electrochemical and liquid phase window, and their ability to solvate high concentrations of metal ions of interest, making them an attractive alternative to conventional electrolytes such as cyanide-based electrolytes, which in addition pose toxicity issues. Use of RTILs in these systems is anticipated to enable deposition or stripping of thin films with specific optical, emissive and electrical properties that may be tuned as needed for specific commercial applications. Furthermore, use of RTILs is anticipated to be beneficial in space applications compared to conventional electrolytes due to their low volatility, preventing evaporation if exposed to vacuum, and their excellent chemical and thermal stability, which may enhance tolerance to orbital radiation.

Electrodeposition of electrochromic materials should be a less expensive alternative to achieve thin metallic films when compared with other deposition technologies, such as thermal spray or vapor deposition. Furthermore, 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.

The present work explores the use of RTIL as electrolytes for reversible silver deposition systems. The first studies utilizing ionic liquid electrolytes for silver deposition focused on chloroaluminate melts, which have a high sensitivity to water, including atmospheric moisture. The present work focuses on deposition and stripping of silver thin films from RTIL with relative low moisture sensitivity. Simple cells composed of transparent electrically conductive electrodes were used and evaluated as a function of the different doping elements (i.e. tin oxide, platinum and silver). In this work, the system is evaluated in terms of RTIL electrolyte, substrate material and operating conditions including pulse/pulse reverse electric fields using cyclic voltammetry and chronoamperometry techniques. These studies were conducted in terms of cycling efficiency and reproducibility. Careful study of the applied voltage was conducted as voltages that are too high damage the transparent electrically conductive electrodes and limit device lifetime and voltages that are too low result in poor mirror formation and low stripping efficiencies. Deposits plated under different conditions were analyzed and compared with an optical surface profilometer, specifically to assess the continuity and smoothness of the resulting silver deposits as a function of deposition conditions. By optimizing the deposit properties, it is anticipated that device lifetime will be extended increasing device performance and associated cost metrics over conventional operation based on constant voltage or constant current electric fields.

Acknowledgements –

Funding for this work is gratefully acknowledged from Air Force SBIR Grant Number FA9453-16-M-0466. Platinum coated ITO electrodes were prepared by Dr. John Bryan Plumley.