High-Performance Phase-Change Materials Based on Paraffin and Expanded Graphite for Solar Thermal Energy Storage

A tradeoff between high thermal conductivity and large thermal capacity for most organic phase change materials (PCMs) is of critical significance for the development of many thermal energy storage applications. Herein, unusual composite PCMs with simultaneously enhanced thermal conductivity and thermal capacity were prepared by loading expanded graphite (EG) after natural aging into the paraffin matrix via an integrated blending method for the first time. Of special interest is that the composite PCMs with an EG load as low as 4 wt % exhibited 642% thermal conductivity (4 wt % EG) and 5% (melting) or 7% (freezing) thermal capacity (1 wt % EG), larger than those of pure paraffin. The characterization results revealed that the short wormlike EG rods built a flexible framework in the paraffin matrix during blending, among which smaller exfoliated graphite flakes were cross-linked in space; thus, a highly effective thermal conductive pathway was constructed. Additionally, the alkylated EG surface after natural aging with high lipophilicity contributed to the good paraffin/EG interface compatibility because of similar chemical compositions and the same polarities of paraffin molecules and the EG surface and thus reduced the interface thermal resistance. Meanwhile, the least EG load in paraffin ensured the highest thermal storage density in the whole system. Under this premise, the increased paraffin crystallinity and the strong intermolecular interactions between paraffin and functionalized EG finally resulted in the enhancement of thermal capacity of the composite PCMs. This work provides a new strategy to prepare high-performance PCMs that are available in the real solar thermal storage applications.