High free volume polymers of intrinsic microporosity for efficient solar thermal fuels with enhanced energy storage capacity

Abstract

Anchoring photoswitches on high free volume polymers is an emerging strategy to prepare high-performance solar thermal fuels (STFs), however, it still lacks a deep understanding about the relationship between free volume and energy storage performance. Herein, a bottom-to-up strategy was developed to prepare azobenzene-based STFs based on polymers of intrinsic microporosity (Azo-PIMs), in which free volumes could be changed by controlling the feeding ratio of co-monomers. Azo-PIM-1 and Azo-PIM-2 were prepared to have 7.41% and 19.07% azo grafting densities, respectively. The fractional free volume (FFV) and specific surface areas of Azo-PIM-1 are 0.17 and 552.8 m2 g−1, respectively, which are higher than that of Azo-PIM-2 (0.15 and 335.5 m2 g−1) due to the lower azo grafting density. The energy densities (EDg) of Azo-PIM-1 and Azo-PIM-2 in cis isomers are 13.39 J g−1 and 36.75 J g−1, and the corresponding storage enthalpy (ΔH) are 88.53 kJ mol−1 and 103. 35 kJ mol−1, respectively. Their ΔH are higher than that of the azo monomer of 83.76 kJ mol−1, demonstrating that PIMs as templates can efficiently increase the stored energy per azo moiety. Meanwhile, the higher ΔH of Azo-PIM-2 than that of Azo-PIM-1 indicates that the reduced free volume can enhance the energy storage capacity by prohibiting the rotation of azo photoisomerization. This work provides a basis for understanding the role of free volume on the energy storage performance of polymer-based STFs.