Synthesis and characterization of silica-encapsulated phase change materials derived from waste cooking oil

Improper disposal of waste cooking oil (WCO) leads to water and soil contamination and greenhouse gas emissions. Its conversion into high-value materials therefore represents a sustainable and circular alternative aligned with renewable energy objectives. This study proposes a green strategy for the development of phase change materials (PCMs) for thermal energy storage (TES), replacing conventional paraffin-based cores with a bio-derived alternative. WCO was converted into hydrogenated fatty acid methyl esters (H-FAMEs) via transesterification and full hydrogenation, producing a crystalline and thermally stable PCM precursor. The H-FAMEs were encapsulated within SiO2 shells via a sol-gel process using tetraethyl orthosilicate (TEOS), and several surfactants were tested to optimize emulsion stability and encapsulation efficiency. Sodium stearate resulted in the highest synthesis yield (57%) owing to its strong molecular affinity with the stearic-acid-rich H-FAME core. The H-FAMEs exhibited a melting temperature of 35.3 ◦C and a latent heat of 195.9 J/g, while the SiO2- encapsulated PCM retained favourable values (Tₘ = 33.9 ◦C; ΔHₘ = 82.4 J/g) with an encapsulation efficiency of 43%. The SiO₂ shell offers rapid energy transfer, resulting in an overall PCM thermal conductivity of 0.65 ± 0.05 W/m⋅K at 20 ◦C. The microcapsules showed an average diameter of approximately 1.0 μm, with limited aggregation and good dispersibility. Differential scanning calorimetry demonstrated excellent thermal reliability, with negligible hysteresis after 50 cycles. FTIR, SEM, and XRD analyses verified complete hydrogenation and successful encapsulation, highlighting the potential of WCO-derived H-FAME@SiO₂ systems as sustainable, paraffin-free PCMs for TES applications.