The rapid growth of solar energy has led to a significant increase in photovoltaic (PV) panel installations worldwide. However, as these panels reach the end of their operational life, managing PV waste presents both a challenge and an opportunity. Among the valuable materials embedded in PV panels, silver stands out due to its crucial role in electrical conductivity and high market value. Recovering silver from end-of-life (EOL) solar panels is essential to enhance resource sustainability, reduce dependency on raw material extraction, and support the circular economy.
Electrometallurgical techniques, particularly electrowinning, have been widely employed for extracting metals in their elemental forms. Researchers have explored multiple methods to enhance the efficiency of silver recovery through electrolysis.
Yang et al. introduced a sustainable approach to silver recovery from discarded solar cells using methanesulfonic acid (MSA) with an oxidizing agent. MSA is preferred due to its high solubility for metal salts, excellent conductivity, and low toxicity. However, its high viscosity can hinder silver ion mobility, reducing electrochemical efficiency. The introduction of water lowers viscosity, improving cathode efficiency and silver deposition rates.
Zhang and Su developed an electrochemical method for silver recovery. The process begins by immersing solar cells in sodium hydroxide for two hours to remove the aluminum layer. The aluminum-free solar cells are then dissolved in a 7:1 volume ratio of MSA and hydrogen peroxide at 75°C for five hours. The resulting silver-containing solution undergoes electrolysis at 0.65 V and a current density of 25 mA, achieving a silver recovery rate of 95%. Residual silicon is extracted using hydrofluoric acid, making the process highly efficient.
Meng and Wen-hsi proposed a different electrowinning technique, where silver is leached from silicon wafers using 11.4% nitric acid at 60°C. The resulting solution undergoes sedimentation to remove tin, followed by electrowinning, in which silver deposits at an applied voltage of 0.35-0.8 V vs. SHE. This method achieves an 87% recovery rate with 99% silver purity. Unreacted solar cell material is further processed using sodium hydroxide, with the resulting sodium nitrate solution repurposed for agricultural fertilizer.
Kanellos et al. explored microbial fuel cells (MFCs) for silver recovery. After leaching silver nitrate from solar cells using nitric acid, the solution is treated in an MFC under anaerobic conditions. Silver acts as an electron scavenger, preferentially reducing itself before other metals, leading to an 86% high-purity silver crystal recovery.
Several alternative techniques have been proposed to improve the recovery of silver from photovoltaic (PV) panels. One promising method is ultrasound-assisted chemical treatment, which utilizes ultrasound at a frequency of 80 kHz to enhance the efficiency of silver etching. This process aids in the removal of anti-reflective layers and silver grid lines. The silver components that are recovered through this technique can then be repurposed for use in electrodes or conductive adhesives.
Another innovative approach involves the use of deep eutectic solvents (DES). This "green" solvent, composed of choline chloride, urea, and CuCl2, has shown impressive results, achieving a silver recovery rate of 98% over multiple cycles, demonstrating its recyclability and effectiveness.
Additionally, photodeposition using ZnO catalysts offers an alternative method for silver recovery. In slightly acidic conditions and with the assistance of UV light, this technique can extract silver from silver chloride solutions. Electrostatic separation is also an effective technique for recovering silver. By separating conductive and non-conductive materials from crushed PV panels, this method achieves high metal concentrations, particularly silver, with an efficiency rate of 87.7%.
Lastly, heat treatment can be employed, where crushed solar panel powder is subjected to temperatures ranging between 660°C and 1000°C. This process allows for the sequential separation of aluminum, silver, and polysilicon. Although highly effective, it does require a significant input of energy.
Current silver recovery processes from EOL PV panels remain energy-intensive, particularly those involving high-temperature pyrolysis and heat treatments. To enhance sustainability, alternative approaches such as microwave-assisted or ultrasonic-enhanced reactions should be explored to lower energy consumption.
Waste by-product management from leaching processes also requires improvement. Techniques such as fractional distillation for solvent recovery can reduce chemical waste. Additionally, recovering secondary metals like copper and aluminum through electrowinning can further optimize resource efficiency.
Government subsidies, tax credits, and public-private partnerships are critical in making PV recycling technologies more economically viable. Strengthening recycling infrastructure and fostering international cooperation can help address regional challenges associated with PV panel waste management.
With the projected increase in PV waste by 2040, implementing efficient silver recovery methods is essential to ensuring material sustainability. While numerous silver recovery techniques exist, each method has its limitations, from energy consumption to hazardous waste generation. The most widely used nitric acid-hydrogen peroxide method, for example, remains costly and poses environmental concerns due to the release of NOx and HF gases.
Future research must focus on refining these processes to enhance economic feasibility, minimize environmental impact, and close the loop on solar panel recycling. Improving silver recovery rates from EOL panels can significantly reduce reliance on newly mined silver, ensuring a more sustainable and resource-efficient approach to PV technology recycling. Without advancements in this sector, PV waste could undermine the sustainability promises of solar energy, emphasizing the urgent need for innovation in recycling methodologies.