Two-step process turns end-of-life PV backsheets into supercapacitor electrodes

A research team in China has developed a two-step technique to recycle Tedlar/polyethylene terephthalate/Tedlar (TPT) PV backsheets into hierarchical porous carbon for use as supercapacitor electrodes.

“Our experiment designed and implemented an innovative two-step activation strategy for effectively treating waste TPT backsheet materials,” co-author Zijun Zhang told pv magazine. “When the backsheet is pyrolyzed, a specific form of sodium source is added. Through in-situ fixation, fluorine is effectively captured, while the process also promotes the conversion and utilization of the carbon source.”

The researchers tested the resulting material as an electrode in laboratory cells and a symmetric supercapacitor.

In the first activation step, the researchers cut waste TPT backsheets into pieces and mixed them with a sodium hydroxide (NaOH) solution at different mass ratios. They stirred and heated the mixtures before drying them. The resulting solids were then pyrolyzed under a nitrogen atmosphere at 700 C for 1.5 hours. This step simultaneously fixed the fluorine, pre-carbonized the polymer, and produced an amorphous hard-carbon precursor with an initial mesoporous structure.

In the second activation step, the researchers ground the precursor and mixed it with solid potassium hydroxide (KOH) at different mass ratios. They then heated the mixtures under nitrogen at temperatures ranging from 600 C to 800 C for 1.5 hours. The resulting material was washed with hydrochloric acid and deionized water and then dried, yielding hierarchical porous carbon with interconnected micro-, meso-, and macropores.

The researchers characterized the carbon materials using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy, and nitrogen adsorption-desorption measurements. The analyses were used to assess crystal structure, graphitization, chemical composition, morphology, specific surface area, and pore-size distribution.

The team also measured fluorine content before and after treatment to determine the fluorine-fixation efficiency. It assessed electrochemical performance using cyclic voltammetry, galvanostatic charge-discharge measurements, impedance spectroscopy, and long-term cycling tests. The researchers then used the best-performing material to assemble a symmetric supercapacitor in a coin-cell configuration and demonstrated it by powering a commercial green LED.

Nitrogen adsorption analysis showed that activation temperature and alkali dosage had significant effects on pore structure and specific surface area. The HPC1 sample, prepared at 700 C, achieved a specific surface area of 1,003.4 m2/g and exhibited a combined microporous and mesoporous structure.

HPC1 achieved a specific capacitance of 469 F/g at a current density of 0.5 A/g in a 6 M KOH electrolyte, while also demonstrating good rate capability, according to the researchers.

The process achieved a fluorine-fixation efficiency of up to 96.5%. After 10,000 charge-discharge cycles at 10 A/g, the material retained 92.4% of its initial capacitance. The symmetric supercapacitor assembled with HPC1 achieved an energy density of 50.2 Wh/kg at a power density of 750 W/kg and successfully powered an LED.

“The results show that in the comprehensive tests of the three-electrode system and symmetric supercapacitors, this material exhibits outstanding electrochemical performance and good cycling stability,” the academics concluded. “This experiment not only solved the environmental pollution problem that might be caused by fluorine elements in discarded TPT backsheet, but also achieved the efficient conversion and high-value utilization of carbon resources.”

The researchers presented their findings in “Recycling photovoltaic backsheet into porous carbon for supercapacitor applications,” published in the Journal of Energy Storage. The research team included scientists from Nanjing Normal University and China Resources Recycling Group New Energy Technology in China.