Electrochemical Separation of Copper and Tin for Silicon Solar Module Recycling
Authors: Adcock Randall , Click Natalie , Chen Theresa , Tao Meng 
Published: December 2024

Silicon solar modules are the most common form of solar photovoltaic (PV) in the US. With a 30-year lifespan, how they are handled at their end-of-life will be crucial to return valuable materials to the industry and prevent hazardous waste from being released into the environment. Current solar recycling technologies often only aim for the recovery of silver (Ag), and less valuable metals like Cu (Cu) and Sn (Sn) are rarely considered. Moreover, hazardous metals like lead (Pb) are also often ignored, even though their buildup in the environment can lead to detrimental effects. Acid leaching is a common method for metals recycling, and the properties of Cu and Sn often lead to them present in the same solution when part of a broader leaching sequence. Thus, this research aims to further understand the separation of Cu and Sn in a hydrochloric acid (HCl) stream in a way that is both circular and maximizes recovery. With the reactions involved, the acid should be consumed during leaching and regenerated during electrowinning, resulting in a circular process that requires minimal chemical input. Electrochemical analysis was performed to determine optimal recovery conditions for both Cu and Sn. Cyclic voltammetry was utilized to determine thermodynamic reduction potentials for both metals. Tafel analysis was used to determine some electrochemical kinetic parameters. Electrochemical impedance spectroscopy (EIS) also allowed the determination of relevant kinetics parameters. Chronoamperometry was used to recovery the metals with a constant applied voltage with respect to a reference electrode (Ag/AgCl). The resulting cathodes were then analyzed with energy dispersive x-ray spectroscopy (EDS) coupled with a scanning electron microscope (SEM). Recovery rates and current efficiencies were determined by concentrations of metal cations in solution through Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Cyclic voltammetry determined the reduction potential peak for Cu and Sn to be -0.44 V and -0.92 V vs. Ag/AgCl respectively. Applied potentials with respect to the Ag/AgCl reference of -0.4 V, -0.45 V, -0.5 V, -0.55 V and -0.6 V for 8 hours were explored for Cu recovery, and their recovery rates were 0%, 8.6%, 82.8%, 84.2%, and 88.5% respectively. The coulombic efficiencies associated with these tests were 0%, 4.5%, 29.6%, 28.2% and 30.0%. Throughout all these tests, no Sn was recovered on the cathode. The Sn remained in solution as determined through ICP-OES. From a solution after Cu electrowinning, -1.0 V was applied for Sn recovery. The recovery rate for this was 100% with a coulombic efficiency of 14.5%. The cathodic deposits revealed >99% pure Cu and Sn during their respective recovery steps. Therefore, it was concluded that Cu and Sn could be electrochemically separated through this method. Through Tafel analysis and EIS, the kinetics parameters for Cu and Sn electrowinning were determined. For Cu, the symmetry factor (α) was determined to be 0.35 and the exchange current density (Io) was determined to be 4.68 mA/cm2. For Sn, α was 0.13 and Io was 26.6 mA/cm2. Solar recycling will be a must in the future circular economy, and developing low cost, chemically circular methods to perform recycling will help to incentivize scaling these processes to a point where they are useful. This research provides a method of separating Cu and Sn as pure components for recycling solar panels.