Novel bio-based composite phase change materials with reduced graphene oxide-functionalized spent coffee grounds for efficient solar-to-thermal energy storage

Abstract Spent coffee grounds (SCGs), a kind of highly recyclable and useable biomass, are often ignored, discarded, and further become a serious environmental threat. In this work, effective high-value utilization of SCGs in the field of composite phase change materials (CPCMs) was proposed. SCGs with the introduction of 1–5 wt% reduced graphene oxide (RGO) were used as supporting material to stabilize the shape of the CPCMs to solve problems of leakage and low light-to-heat conversion. RGO-modified SCGs (SCGs@RGO) have a better three-dimensional porous structure than pristine SCGs, therefore absorbing 60.3 wt% PEG and having excellent leakage-proof performance and showing slight thermal conductivity enhancements. SCGs@RGO has strong absorption in the full UV–Vis–NIR range, hence it exhibits extremely strong light-to-heat conversion capacity and stores solar thermal energy in a short time (245 s) of light. Besides, PEG/SCGs@RGO CPCMs show good reusability, chemical stability, crystalline stability, and excellent thermal stability in phase change tests up to 100 cycles.