Electrochemical Characterization of Silver and Iron Ions in Choline Chloride-Ethylene Glycol DES Electrolyte
Authors: Jahrsengene Goril , Wang Zhaohui , Martinez Ana Maria 
Published: December 2024

Solar energy is growing to be an important source of renewable energy in the world, and the need of a recycling process to recover the strategic materials used for this technology is crucial. A route to recover Si and Ag from end-of-life photovoltaic (PV) cells as well as scrap from PV manufacturing using a Deep Eutectic Solvent (DES) have been proposed and developed as part of the PHOTORAMA project. The suggested ionometallurgical route uses a DES solution with FeCl3 as oxidizing agent to leach silver and separate it from Si, before recovering Ag metal by electrowinning (EW) from the DES-based electrolyte.
Choline Chloride (ChCl)-Ethylene Glycol (EG) DES mixtures, often referred to as ethaline, in composition 1:2 molar ratio was identified as the preferred DES-system to be used for leaching and subsequent EW. As such, the DES leachate used as electrolyte in the EW step, should ideally consist of Ag(I) and Fe(II) dissolved in the DES. The goal of this work was to carry out electrochemical characterization of the electrochemical systems and understand if the presence of soluble iron species may affect the silver deposition.
Model electrolyte solutions were produced by dissolving AgCl, FeCl2, and FeCl3·6H2O in the DES mixture. Electrochemical investigations were carried out at 60 °C by cyclic voltammetry (CV) and chronoamperometry (CA) using a 3-electrode setup. Moreover, silver deposits were obtained by potentiostatic or galvanostatic electrolysis, and subsequently analysed by Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS).
Cyclic voltammograms obtained using a Pt-wire with a scan rate of 200 mV/s, show well defined Ag(I)/Ag and Fe(III)/Fe(II) electrochemical systems. The deposition of metallic Fe was found to be close to the cathodic decomposition of the DES electrolyte. High quality silver deposits were obtained by choosing optimal deposition potentials or current densities. SEM-EDS analysis of the silver deposits obtained on Pt, silver, and stainless steel (316L) substrates showed a nice, dense layer of Ag crystals with a microstructure of truncated octahedrons, with a purity of 96-98 wt%. The impurities were identified as C, O or Cl and probably stemmed from the DES electrolyte remains.
Based on the electrochemical characterization of the DES systems, and the analysis of the obtained silver deposits, the concept proposed for the recovery of metals in the PHOTORAMA project was found to be suitable. Water added to the DES (up to 10 wt%), which may be stemmed from the crystal water of the oxidant added, or absorbed from air, gave improved kinetics. The presence of excess of Fe(III), which could arise from incomplete leaching reaction, had the obvious negative effect on the current efficiency of the Ag(I) reduction reaction, but had no effect on the silver morphology or purity of the obtained deposit.
Acknowledgements
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 958223. The content in this publication reflects only the views of the authors. The European Commission is not responsible for any use that may be made of the information it contains.

Figure 1