Electrodeposition of Cu, Co and Re from Water in Salt Electrolytes
Authors: Huang Qiang , Sides William D. , De Shuvodeep , Hu Yang 
Published: February 2020

Electrodeposition of Cu, Co and Re from Water in Salt Electrolytes

Qiang Huang,William D. Sides, Shuvodeep De, Yang Hu

Department of Chemical and Biological Engineering, University of Alabama, Tuscaloosa, AL 35487

Water-in-salt electrolytes, aqueous electrolytes with a super high concentration of salt, provides a unique electrolyte environment for electrodeposition, where free water molecules are depleted by the hydration of salt. A 21 M lithium bis(trifluoromethane sulfonyl)imide (LiTSFI) aqueous electrolyte has been recently demonstrated to enable the lithiation and delithiation in an aqueous lithium ion battery system[1]. The super highly concentrated LiTSFI resulted in a much-widened electrochemical window, not only by suppressing the hydrogen evolution reaction rate but also by forming in a fluorinated passivation layer at the electrode-electrolyte interface. This concept has been applied to the electrodeposition of superconducting rhenium, where high concentration of LiCl suppressed the proton reduction and improved the film morphology and superconductivity stability[2]. This talk reports a systematic study on the electrochemical behavior of three transition metals, Cu, Co and Re, during the deposition in water-in-salt.
The excessive chloride anion in the water-in-salt electrolyte is found to stabilize cuprous ion and separates the cupric ion reduction and copper deposition. Detailed CV at the first reduction step reveals that the cupric ion reduction is a highly reversible single electron transfer process. Systematic analyses were carried out using stationary cyclic voltammetry and linear sweep voltammetry on rotating disc electrodes to determine the diffusion coefficients as well as the effective concentration of cupric ions and protons in the water-in-salt electrolytes. The diffusion coefficient of proton significantly decreases, for over 20 times, upon the addition of 8 M LiCl, whilst the diffusion of cupric ion merely decreases for less than 2 times. This is consistent with the “proton hopping mechanism” in proton diffusion in water, where the high concentrated salt disrupts the hydrogen bond network between water molecules and suppresses such hopping mechanism. Furthermore, the high concentration of LiCl seemed to increase the effective concentration of proton in the electrolyte. A comparison between Li, Na and K chloride salts is shown in Figure 1(b). Li cation is more effective than the larger Na+and K+in interacting with the water molecule dipole, disrupting the hydrogen bond network, suppressing the proton hopping, and at the same time increasing the acidity of water molecules.
Copper electrodeposition is carried out in LiCl water-in-salt electrolyte and the partial current density is determined using anodic stripping techniques. As shown in Figure 1(c), the deposition rate reaches mass transport limit current at -0.4 V, decreases slightly upon the limiting current of proton reduction, and rapidly decreases to 0 on the commence of water reduction at -1.5 V. This suggests a surface blocking by adsorbed hydrogen intermediates in proton reduction and a surface passivation at highly negative voltage – probably due to the formation of a hydroxide species. This is found different from the partial current decrease observed in rhenium deposition shown in Figure 1(d), which occurs at a more positive potential of -0.9 V before water reduction starts. A further reduction of Re metal into rhenide anion (Re-) was proposed to occur at this potential, resulting in the decrease of overall deposition rate.

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Figure 1