Complete utilization of waste pomegranate peels to produce a hydrocolloid, punicalagin rich phenolics, and a hard carbon electrode

This work demonstrates the complete utilization of waste pomegranate peels (WPP) to produce pectin, punicalagin rich phenolics, and a hard carbon electrode. The high yields of pectin (24.8%) and phenolics (11.9%) were obtained at optimal ultrasound (20 min ultrasound time, 15 mL g^-1 liquid:solid ratio) and cellulase treatment (55 U g^-1 cellulase dosage, 4 h treatment time) of WPP. The IR, ¹H NMR, and TGA analysis showed that the recovered pectin was similar to commercial pectin. The recovered pectin had a 68.5% degree of esterification, 146.5 kDa molecular weight, and 72% galacturonic acid content. A high content of valuable punicalagin (71.2% of total phenolics) occurred in the recovered phenolics. The WPP leftover (after pectin and phenolics recovery) was carbonized via pyrolysis and then impregnated with potassium hydroxide and heated in an inert atmosphere to obtain hard carbon (HC) to be employed as an electrode in electrochemical cells. The Raman spectroscopy, X-ray crystallography, and CHN analysis confirmed the formation of HC which was further characterized by scanning electron microscopy. During the electrochemical testing with a sodium battery, the electrode fabricated with HC provided a constant charge-discharge cycle performance of 180 mA h g^-1 at a C/2 rate and nearly 100% Columbic efficiency after 100 cycles using a Na-salt electrolyte.