An Electrochemical Process for the Recovery of Metals and High Purity Silicon from Photovoltaic Modules

The increasing number of photovoltaic (PV) panels installed worldwide requires solutions for their disposal at their end-of-life. PV modules contain several valuable metals like copper and silver as well as toxic ones like lead used in solder. Another valuable component are silicon wafers, because of their high purity, which is obtained in energy and resource intensive production processes. Many recycling procedures, however, are downcycling processes that do not recover these components, but rather are based on shredding and landfilling the modules or obtaining only one of those materials. Often, nitric acid is used as leaching agent for the recovery of silver and copper resulting in harmful NOx emissions that necessitate additional gas treatment. We present an alternative electrochemical process by which leaching reagents are generated on boron-doped diamond electrodes as anode material, which are ideal for this task due to their high overpotential for the oxygen evolution reaction. By evaluating different electrolytes, sulfuric acid was found to be suitable by generating peroxydisulfate (S₂O₈²⁻). With its standard redox potential of +2.0 V, it is possible to oxidize rather noble metals like silver. To evaluate the efficiency of the process S₂O₈²⁻ was generated from sulfuric acid in an H-cell setup and the leaching rate of copper, silver and tin was investigated and optimized by varying acid concentration and current density. Efficient leaching of all metals was achieved in 5 M sulfuric acid at 200 mA/cm². The feasibility of the process for real waste streams was demonstrated by disassembling PV modules and leaching busbars and silver coatings from wafers. The complete removal of the metal coatings from the residual wafers was confirmed by EDX. A significant advantage of the process with S₂O₈²⁻ as leaching agent is, that the reaction with metals forms sulfate that can be oxidized again within a cyclic process driven by electricity as displayed in figure 1. Additionally, the metals can be recovered by electrowinning from the etching solution within the same cell. This was demonstrated by recovering Ag, Cu and Sn from sulfuric acid solution. By using potentiostatic conditions it was additionally possible to separate the metals by consecutively plating them in the order of their respective redox potentials. Leached metals from PV waste were recovered electrochemically with 99 % and 88 % yield for Cu and Ag, respectively. Sn could not be plated due to the high amounts of persulfate present in solution, however recovery by precipitation is possible. Pb is obtained as undissolved residue during leaching, due to the extremely low solubility of PbSO4 and PbO2. Aside from the H-cell experiments the generation of S₂O₈²⁻ and electrowinning was successfully investigated in a flow cell setup. This system benefits from a significantly decreased cell voltage making the process more energy efficient and allows easier upscaling towards industrial applications. The presented process is energy efficient as both electrode reactions are utilized and furthermore does not require the continuous addition of chemicals, therefore, making it an eco-friendly and sustainable alternative to conventional recycling methods. Figure 1: Scheme for the cyclic process of peroxydisulfate generation, leaching and electrowinning during the electrochemical recycling of PV waste. Figure 1