Until now, solar thermal energy storage within phase-change materials (PCMs) depends on π–π and van der Waals interactions to improve their miscibility, which often brings about finite enhancement of energy density. Here, we report one nanoconfinement approach to improve energy density using microphase separation of a block copolymer, allowing PCMs to selectively disperse into nanodomains of polymer film for use as solar thermal fuel (STF). Upon doping a certain amount of photoinert organic PCMs with liquid-crystalline block copolymer containing azobenzene as the continuous mesogenic phase, the periodic nanostructures remained due to the microphase separation, which also brings about a distinct nanoconfinement effect for the dopant PCM nanodomains, achieving crystallization temperature from 24 to −38 °C. This unique feature originates from the nanoscale microphase separation, enabling natural sunlight and environmental heat to be stored simultaneously, which can be released in several ways: cis-to-trans isomerization and isotropic-to-liquid crystal phase transition of the azobenzene-containing polymer block at a higher temperature and phase transition of PCMs at a lower temperature, even below −30 °C. Due to the improved STF performance, wearable warm fabrics and photothermal pneumatic actuators were successfully obtained. The present work paves the way for developing high-performance STF systems achieved by microphase separation (MPS) and nanoconfinement, facilitating the advancement and applications of STF materials in the fields of wearable smart materials, warm fabrics, and actuators.

Molecular solar thermal fuels (MOSTs), especially azobenzene-based MOSTs (Azo-MOSTs), have been considered as ideal energy-storage and conversion systems in outer or confined space because of their “closed loop” properties. However, there are two main obstacles existing in practical applications of Azo-MOSTs: the solvent-assistant charging process and the high molar extinction coefficient of chromophores, which are both closely related to the π–π stacking. Here, we report one efficient strategy to improve the energy density by introducing a supramolecular “cation−π” interaction into one phase-changeable Azo-MOST system. The energy density is increased by 24.7% (from 164.3 to 204.9 J/g) in Azo-MOST with a small loading amount of cation (2.0 mol %). Upon light triggering, the cation−π-enhanced Azo-MOST demonstrates one gravimetric energy density of about 56.9 W h/kg and a temperature increase of 8 °C in ambient conditions. Then the enhanced mechanism is revealed in both molecular and crystalline scales. This work demonstrates the huge potential of supramolecular interaction in the development of Azo-MOST systems, which could not only provide a universal method for enhancing the energy density of solar energy storage but also balance the conflicts between molecular design and the condensed state for phase-changeable materials.

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