Long-Term Solar Energy Storage under Ambient Conditions in a MOF-Based Solid–Solid Phase-Change Material

This paper demonstrates a metal–organic framework (MOF) containing photoswitches within the pores as a hybrid solar thermal fuel (STF) and solid–solid phase-change material (ss-PCM). A series of azobenzene-loaded MOFs were synthesized with the general formula Zn2(BDC)2(DABCO)(AB)x (BDC = 1,4-benzenedicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]octane, AB = azobenzene, where x = 1.0, 0.9, 0.5, 0.3), herein named 1⊃AB1.0, 1⊃AB0.9, 1⊃AB0.5, and 1⊃AB0.3 respectively. X-ray powder diffraction, solid-state NMR, and density functional theory calculations were used to explore in detail the structural changes of the host framework that take place upon loading with the AB guest molecules. Differential scanning calorimetry measurements reveal a reversible phase change, which is absent from the evacuated framework. Upon irradiation with 365 nm light, 40% of the AB guests converted from the trans to the higher-energy cis isomeric form in 1⊃AB1.0. The energy stored within the metastable cis isomers is released upon heating and balances the endotherm associated with the phase transition. However, the exotherm associated with the phase transition is retained upon cooling, resulting in a net energy release over a full heating–cooling cycle. The maximum energy density is observed for the fully loaded composite 1⊃AB1.0, which releases 28.9 J g–1. In addition, the cis-AB guests in this composite showed negligible thermal reconversion during 4 months at ambient temperature, with an estimated energy storage half-life of 4.5 years. Further development of MOF-based STF-ss-PCMs could lead to applications for solar energy conversion and storage, and thermal management.