Manufacturing of a KOH-functionalized LiCoO2-rich Nano catalyst from spent lithium-ion batteries for microalgal biodiesel production

Abstract

The circular use of end-of-life lithium-ion battery cathodes as catalysts can connect electronic-waste management with renewable-fuel production. This study extends previously reported route-selection and cathode-nanosizing work by applying a KOHfunctionalized LiCoO₂-rich recovered oxide to the transesterification of Chlorella vulgaris microalgal oil. Spent 18650 cells were discharged and dismantled; the cathode fraction was subjected to direct treatment, calcination at 700 °C, staged milling, citricacid/H₂O₂ leaching, and alkaline functionalization. Box-Behnken response-surface designs with 27 runs each were used to optimize ball-milling and biodiesel-production variables. At 60 min, 5 mm ball diameter, a 30:1 ball-to-material ratio, and 650 rpm, the experimental particle sizes were 135.3 nm before leaching and 117.5 nm after leaching, with validation errors below 5%. X-ray diffraction retained LiCoO₂-related reflections, while DLS and FESEM showed a nanoscale fraction with residual agglomeration. Biodiesel yield varied from 56.82% to 98.84%. The quadratic model was significant (p < 0.0001), the lack of fit was non-significant (p = 0.2398), and the experimental-predicted pairs gave R² = 0.965. Catalyst loading and reaction time were the dominant main effects. A model-recommended confirmation condition of 1.717 wt.% catalyst, an 8.51:1 methanol-to-oil molar ratio, 3 h, and 57.5 °C produced 95.17% yield against 99.073% predicted. The product density (0.8819 g cm⁻³), viscosity (5.403 cSt), and flash point (138.3 °C) satisfied ASTM D6751 limits; viscosity was slightly above the EN 14214 upper limit. The reuse yields decreased from 95.17% (cycle 1) to 60.30% (cycle 5), indicating that regeneration or better active-site retention is needed after the third cycle.