A rechargeable molecular solar thermal system below 0 °C.

Area of Science:

• Materials Science
• Photochemistry
• Energy Storage

Background:

• Optimal temperature control is vital across diverse applications, including chemical processes, electronics, and climate regulation.
• Photochemical molecular thermal energy storage systems (MOST-PCMs) present a promising approach for capturing and managing thermal energy.
• Existing MOST-PCMs often face limitations in operating temperature range and energy density.

Purpose of the Study:

• To design and synthesize visible-light-responsive azopyrazoles capable of integrating MOST and phase change materials (PCMs).
• To demonstrate energy capture and release below 0 °C using light stimuli.
• To achieve high energy density by co-harvesting light and ambient thermal energy.

Main Methods:

• Synthesis of visible-light-responsive azopyrazole compounds.
• Integration of azopyrazoles with phase change materials to create MOST-PCMs.
• Characterization of energy storage and release properties under controlled light irradiation and temperatures.
• Fabrication of photo-controlled transparent coatings for thermal regulation demonstration.

Main Results:

• A series of azopyrazoles were successfully coupled with PCMs for energy storage below 0 °C.
• The MOST-PCMs were charged using blue light at -1 °C and discharged heat using green light.
• High energy density of 0.25 MJ kg^-1 was achieved through combined light and thermal energy harvesting.
• Demonstrated photo-controlled transparency coatings achieved a temperature difference of up to 22.7 °C against sub-zero surroundings.

Conclusions:

• The developed azopyrazole-based MOST-PCMs effectively store and release thermal energy below 0 °C using light triggers.
• This approach enables co-harvesting of solar and ambient thermal energy, leading to high energy density.
• The study provides molecular design principles for advanced MOST-PCMs applicable to a wide temperature range, including sub-zero conditions.