A. Mukhopadhyay, M. Shi, H.W. Rollins, D.M. Ginosar
Idaho National Laboratory,
United States
Keywords: photovoltaics, solar panel, end-of-life recycling, tellurium, metal recovery, electrochemistry
Summary:
To achieve global decarbonization goals by 2050, as much as 100 TW of solar power capacity will be required globally. Low cost, high efficiency, thin-film cadmium telluride (CdTe) photovoltaics (PV) currently represent >50% of new commercial scale solar panel installations in the United States. A resilient supply chain of tellurium (Te), a critical semiconductor element, is needed to support such growth. By 2050, the United States is projected to produce 10 Mt of PV waste containing 2,010 t of Te. Te is as rare as platinum in the earth’s crust (~ 1 microgram /kilogram) and its production is geographically limited. End-of-life (EOL) CdTe PV solar panels can be a significant secondary source of Te. Recycling that waste will support a resilient Te supply chain. The current industrial process for recycling EOL CdTe PVs relies on oxidative leaching using hydrogen peroxide (H2O2) and sulfuric acid (H2SO4). The process extracts metals efficiently, but the carbon footprint is high due to the sacrificial use of the oxidant H2O2 and the neutralization of acidic wastewater. In this study, we developed a novel recycling technology based on electrochemical (EC) leaching for EOL CdTe PVs with lower cost, lower chemical and energy consumption, safer operation, and reduced waste generation. In our approach, H2O2 is electrochemically generated onsite from oxygen supplied as air using renewable electrical energy. EC leaching allows recovery of Te and Cd with near-quantitative extraction efficiencies (>99%) under ambient conditions. Process optimization elucidated the impacts of current, time, feed gas composition, and feed gas flow rate on metal extraction efficiency. EC leaching rates for Te and Cd are 5- to 10-fold faster than traditional chemical leaching rates. Furthermore, EC leaching eliminates the need of 40 kg of H2O2 / t of EOL CdTe PV, decreases H2SO4 needs by 8-fold, and reduces chemical base needs for wastewater neutralization by 8-fold. In addition, EC leaching generates a cleaner byproduct, glass, with much lower Cd contamination (25 ppm) than that in the case of chemical leaching (>200 ppm). We further developed an electrowinning (EW) process to separate, purify, and isolate Te and Cd from the aqueous EC leachate containing mixture of different metal contaminants including iron, selenium, aluminum, copper, zinc, led, tin, etc. EW allowed >75% recovery yield and >80% purity of Te as an isolated metal from the EC leachate. EW process optimization revealed the impacts of cathodic reduction potential and time on metal recovery yield and metal purity. Application of EC leaching and EW for sustainable recovery of Te from EOL CdTe solar panels constitutes the first of its kind example.