A series of imidazolium-based ionic liquids were synthesized by introducing functional groups in the imidazolium cation to develop new phase change materials. The structures of these ionic liquids were determined by nuclear magnetic resonance; the quantum calculation was performed based on density functional theory by Gaussian 09 to determine the number of hydrogen bonds among the ions. The heat of fusion, heat capacity, and thermal storage density of the ionic liquids were investigated by DSC; in addition, the thermal stability was determined by TGA. The thermal analysis results indicate that new functionalized ionic liquids have excellent thermal stability with decomposition temperatures higher than 475 K. In addition, the heat of fusion, heat capacity, and thermal storage density of the functionalized ionic liquids increased on average by 34, 86.5, and 10096, respectively, compared with alkyl chain ionic liquids with the same carbon numbers. These superior properties are attributed to the additional hydrogen bonds in the functionalized ionic liquids.