Reversible Silver Electrodeposition from Boron Cluster Ionic Liquid (BCIL) Electrolytes †,# †,# ‡ ‡ §, Rafal M. Dziedzic, Mary A. Waddington, Sarah E. Lee, Jack Kleinsasser, John B. Plumley, ⊥ William C. Ewing,| Beth D. Bosley,,* Vincent Lavallo,‡,* Thomas L. Peng,§,* Alexander M. Spokoyny.†,∇,* †Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, CA 90095, United States ‡ Department of Chemistry and Biochemistry, University of California, Riverside, 501 Big Springs Rd., Riverside, CA 92521, United States §Air Force Research Laboratory, Kirtland AFB, New Mexico, United States, Albuquerque, NM 87123, United States ⊥Department of Chemistry and Chemical Biology, University of New Mexico, 300 Terrace St. NE, Albuquerque, NM 87131, United States ǁBoron Specialties LLC, 2301 Duss Avenue, Building 9, Ambridge, PA 15003, United States ∇California NanoSystems Institute (CNSI), University of California, Los Angeles, 570 Westwood Plaza, Los Angeles, CA 90095, United States Supporting Information Placeholder ABSTRACT: Electrochemical systems offer a versatile means for creating adaptive devices. However, the utility of electrochemical dep- osition is inherently limited by the properties of the electrolyte. The development of ionic liquids enables electrodeposition in high-vacuum environments and presents opportunities for creating electrochemically adaptive and regenerative spacecraft components. In this work we developed a silver-rich, boron cluster ionic liquid (BCIL) for reversible electrodeposition of silver films. This air and moisture stable elec- trolyte was used to deposit metallic films in an electrochemical cell to tune the emissivity of the cell in situ, demonstrating a proof-of- concept design for spacecraft thermal control. Keywords: Reversible electrodeposition, ionic liquids, infrared transparent electrochemical cell, boron clusters Figure 1 Electrodeposited metallic structures are ubiquitous in modern A B technologies (e. g. batteries, protective coatings, displays, and 0.2 BCIL-1a microchips) and emerging technologies such as resistive memory 0.0 1,2,3 and reconfigurable electronics. The broad utility of -0.2 HCB11H11 1 eq. Ag[HCB11H11] electrochemically deposited structures grows as new electrolytes 2.5 eq. [BMIM]Br -0.4 enable electrodeposition in increasingly challenging environments. Controlled reversible electrodeposition of metallic -0.6 structures offers tunable optical properties and electrical ) 2 conductivity along with the ability to adapt the electrochemical 0.2 BCIL-1b system to changing application needs. This adaptablity is 0.0 appealling in the context of reconfigurable electronic components Figure 1. Left, some decomposition products of commonly used -0.2 which could extend a device’s lifetime through in situ ionic liquid anions. Right, monocarborane anion used in this 1 eq. Ag[HCB11H11] 1.25 eq. [BMIM]Cl work.A B -0.4 restructuring. 1.25 eq. [BMIM]Br non-aqueous electrolytes has enabledSilver safeDissolved electrodeposition of Early electrodeposition methods used aqueous electrolytes -0.6 due to the solubility of common metal ions.1 The development of water sensitive metals for new40 electrochemicalSilver Deposited applications such Current Density (mA/cm as high density energy storage.4 Additionally, electrodepostion from 0.2 BCIL-1c 18% decrease in 0.0 20 total energy transmission -0.2 Transmission (%) -0.4 1 eq. Ag[HCB11H11] 0 Increasing -0.6 2.5 eq. [BMIM]Cl IR Transmission Br- : Cl- 4000 3500 3000 2500 2000 1500 1000 500 Ratio -1 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 Wavenumber (cm ) Voltage (V) Figure 2 A B BCIL-2: ) 2 0.4 1 eq. Ag[HCB11H11] 2.5 eq. [BMIM]Br 0.2 0.2 eq. CuBr2 0.0 -0.2 -0.4 CurrentDensity (mA/cm -0.6 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 Voltage (V) Deposited Dissolved -2 V +0.3 V Electrochemical Cell Schematic Reversible Silver Electrodeposition Figure 2. Formation of BCIL-1 by mixing [BMIM]Br with Ag[HCB11H11]. Inset shows the elementary steps associated with silver electrodeposition in BCIL-1. Representative cyclic voltamogram of BCIL-2, Red and blue arrows indicate cathodic (deposition) and anodic (dissolution) sweep direction respectively. ionic liquids has emerged as a potential alternative to organic properties of spacecraft surfaces. Additionally, the low volatility solvent electrolytes and presented newDeposition opportunities for of IL electrolytes offers a means of performing electrodeposition electrodeposition in high vacuum and hostile environments such in high vacuum environments. Due to its positive reduction as electron microscopes and plasma chambers.5,6 potential, only small volatages are required to electrodeposit silver 1 Ionic liquids (ILs), salts with low melting points (typically from ionic salt precursors. Electrodeposition of silver was <100 ºC), combine the high ion solubilityDissolution of aqueous media, the historcially performed using cyanide-based electrolytes due to the 1-x 1 chemical flexibility of organic solvents, and low volatility and formation of stable Ag(CN)x complexes. However the toxicity wide electrochemical windows achievable with molten salts.7 The of aqueous cyanide baths lead to the development of silver versatility of ILs is increased by tailoring the cation-anion pairs electrodeposition from ILs and organic solvents.4,5 These non- for individual applications.7 Yet, despite the numerous cation- aqueous electrolytes were suitable for reversible silver anion pairs that can form ionic liquids, much of ionic liquid electrodeposition and served as proof-of-concept works for tunability is achieved by modifying the cationic component.7 This electrochemically modulated optical films.8f,15 The reversibility in disparity is largely due to the synthetic availability of cationic these systems is attained from the halide-trihalide redox couples - - 16 - 17 molecular scaffolds such as ammonium, phosphonium, and (X /X3 , X = Cl, Br, or I), and stable [AgX2] complexes. heterocyclic motifs. Conversely, ionic liquids in which the anionic Despite these advances, reversibility in all-ionic electrolytes was component engenders the IL properties are scarce and the anions limited only to a handful of ILs. The majority of these, however, are seldom amenable to additional functionalization.8 still generate unstable, volatile, or corrosive decomposition Furthermore, the relative instability of commonly used IL anions products (Figure 1A).18 In this report, we detail our work on limits the longevity of IL-based electrochemical devices. This creating an electrolyte for silver deposition containing a stable becomes especially problematic when designing systems which anion amenable to additional functionalization. require prolonged operation time. Therefore we sought to develop We developed a salt metathesis reaction between 1-butyl-3- an ionic liquid that contains a highly stable and easily tunable methyl-imidazolium bromide, [BMIM]Br, and silver anionic core. Such “anion-based” ILs could use the metal of monocarborane, Ag[HCB11H11], to generate a boron cluster ionic interest as the cation and enable high metal loadings suitable for liquid, BCIL-1, with high silver content (Figure 2A). The parent rapid electrodeposition. Anionic polyhedral boranes represent a carborane salt can be easily synthesized via a salt metathesis 19 class of modular molecular building blocks suitable for designing reaction between Cs[HCB11H11] and AgNO3 with an 82% yield. “anion-based” boron cluster ionic liquids (BCILs). Their robust This ionic liquid, features an inert ionic matrix of [BMIM]+ and - - boron-cluster framework provides an anionic scaffold amenable to [HCB11H11] ions with [AgBr2] ions acting as the primary redox tuning of melting point, solubility, and redox potential.9,10 In this species (Figure 2A). Dialkylimidazolium cations and work, we set out to develop an all-ionic electrolyte suitable for monocarborane anions were selected due to their electrochemical reversible deposition of silver films. Specifically, we describe an stability and weakly-coordinating characteristics suitable for all-ionic electrolyte to reversibly deposit silver films for the designing an inert ionic matrix.20 The BCILs were enclosed purpose of controlling infrared (IR) emission from an between two FTO electrodes to form transparent, two-electrode electrochemical cell. This technology complements exisiting electrodeposition cell (Figure 2A). One FTO electrode was set as approaches to controling thermal emission from spacecraft the working electrode while the other was set as both the counter surfaces.11,12 Importantly, BCILs represent a critical component and reference electrode. Cyclic voltamograms (CVs) show a allowing these devices to perform reliably without having to reduction peak corresponding to reduction of silver complexes to exclude air and moisture; common contaminants that are form silver deposits at the working electrode (Figure 2 and 3). introduced during electrochemical device assembly that are Electrodeposition begins at -1.5 V vs. FTO with a distinct current difficult to remove. loop occurring in the cathodic region, which indicates a Silver films play an important technological role as electrical nucleation-limited deposition. Reversing the polarity of the conductors and optical coatings. Silver offers unique advantages electrochemical cell toward +0.3 V vs. FTO leads to dissolution of over other transition metals because of its high spectral the silver deposits at the working electrode back into the reflectivity,