Solar thermal power cells enable the on-demand discharge of accumulated chemical energy in the form of heat.
Study: Embedding Azobenzene-Functionalized Carbon Nanotubes into a Polymer Matrix for Stretchable, Composite Solar Thermal Devices. Image Credit: Guenter Albers/Shutterstock.com
A paper published in The Journal of Physical Chemistry C reported the integration of multi-walled carbon nanotubes in a flexible styrene-ethylene-butylene-styrene (SEBS) triblock copolymer framework using an azobenzene derivative and carboxylic acid patterning to construct lightweight and flexible solar thermal batteries.
An Introduction to Solar Thermal Batteries
Over 90% of the primary energy generated globally is used or wasted in the form of heat. It is, therefore, imperative to decarbonize heating systems.
Solar thermal batteries are a possible alternative to heating techniques based on fossil fuels. The solar thermal power cells function on the principle of capturing and storing solar energy, which may then be discharged in the form of heat when needed using solar-activated molecular switches.
Sunlight-powered switches belong to a subclass of solar thermal batteries that use the molecules from the azobenzene family to store light and discharge heat.
Isomerization Mechanism of Azobenzene
Azobenzene has two isomers: the trans isomer and the cis isomer. Steric hindrance among the two phenyl groups causes the cis isomer to occur at a higher energy level than the trans isomer.
When subjected to ultraviolet irradiation, trans to cis isomerization happens through a complicated process that may involve either the azo bond rotation or the inversion of a singular bond.
The shift from trans to cis configuration happens under ultraviolet-A (UVA) light, also referred to as "charging". The transitional state through inversion is more energetic than the transitional state through rotation.
Once azobenzene exists as the cis isomer, its molecules return to the trans conformation through thermal relaxation or photo-initiation.
How Can MWCNTs Help?
Thermal losses in a polymeric matrix can inhibit solar thermal devices with limited heat conductivities from maximizing heat transmission.
The incorporation of azobenzene into SEBS, together with the addition of azobenzene-linked multi-walled carbon nanotubes in the polymeric framework, produces an independent, lightweight, and flexible solar thermal device.
The advantageous thermal qualities of multi-walled carbon nanotubes allow them to operate as a thermal channel while improving the solar spectrum absorption of azobenzene and the mechanical characteristics of the solar thermal device.
The Advantages of Using SEBS
SEBS is stable under thermal and ultraviolet influence, as well as being widely accessible in a variety of styrene concentrations, allowing for specific manipulation of the free volume by altering the stiff and elastic blocks of the polymeric backbone.
Another advantage of SEBS over stiff composite frameworks like aromatic polyamides is that it may be formed into flexible and lightweight devices.
The azobenzene composite is endowed with a modifiable free volume by the flexibility inhibiting bulk styrene monomer elements in SEBS.
The team investigated the kinetics and storage capability of azobenzene in SEBS substrates with different styrene concentrations to obtain a basic knowledge of AB cis-trans isomerization inside a SEBS framework.
They also studied the impact of linking azobenzene and multi-walled carbon nanotubes for forming a percolated thermally conductive channel.
Solar thermal batteries were developed with varying combinations of SEBS, azobenzene, and multi-walled carbon nanotubes. The team then identified the variations in thermal energy storage capacity and mechanical characteristics of the resulting mixtures.
Important Findings of the Study
The kinetics of azobenzene back-isomerization could be altered by covalently attaching the azobenzene to the multi-walled carbon nanotubes.
The speed and extent of back-isomerization could also be adjusted by varying the free volume of the polymeric framework.
To provide customized discharge performance and stabilize the charged condition, the quantity of linked and unlinked multi-walled carbon nanotubes, the styrene concentration of SEBS, and the extra loading of azobenzene could all be altered.
Devices formed with 57% styrene SEBS exhibited a four-fold increment in the first-order isomerization rate of the charged phase with five minutes of irradiation using activating discharging light.
The free volume of the polymeric substrate influenced the storage capability of the developed solar thermal battery. This was evidenced by the reduced photoisomerization performance between charge and discharge cycles when the SEBS framework was altered to include a larger concentration of the copolymer's bulky, rigid styrene phase.
The solar thermal devices showed solution processability from all liquid-phase precursors by drop casting. This highlighted their potential for scalable manufacturing.
Colburn, T. W., Delmastro, A. C., Figueroa, M., Lopez, F., & Cooper, C. B. (2022). Embedding Azobenzene-Functionalized Carbon Nanotubes into a Polymer Matrix for Stretchable, Composite Solar Thermal Devices. The Journal of Physical Chemistry C. Available at: https://pubs.acs.org/doi/10.1021/acs.jpcc.2c03865