Functionalized mesoporous silica as matrix for shape-stabilized phase change materials

Shape-stabilized phase change materials (ssPCM) are used to reversibly store thermal energy while keeping their macroscopic solid shape. A porous matrix, such as mesoporous silica, can be used to obtain ssPCM through impregnation. However, a non-melting interface layer is formed between the matrix and heat storage component, which severely reduces the available pore volume and the energy storage capacity of the composite. We studied functionalization of mesoporous silica as a strategy for engineering the non-melting layer and improving the heat storage of the resulting composites. Stearic acid was used as the active heat storage phase and mesocellular foam silica as the porous matrix. Phenyl and methyl groups were found to increase the nanoconfined phase enthalpy. The non-melting layer volume is similar to unfunctionalized silica, suggesting that methyl and phenyl groups are present inside the non-melting layer. Carboxylic acid functionalization decreases the layer volume, yielding the highest enthalpy of the nanoconfined phase, of 34.5 J/g. This value is 15% higher than the nanoconfined enthalpy of materials containing unfunctionalized silica. Total heat storage values of 130 J/g were obtained for PCM samples containing phenyl or carboxylic acid functionalized matrices. The functionalized materials show high stearic acid loading (72-79 wt%), shape-stability and constant thermal response over 200 heating-cooling cycles. A facile method for assessing the non-melting layer volume is provided. The study shows that suitable surface functionalization can suppress the non-melting layer formation, providing a path for enhancing the heat storage potential of shape-stabilized PCM containing mesoporous silica matrices. (C) 2019 Elsevier Ltd. All rights reserved.