An azobenzene-based photothermal energy storage system for co-harvesting photon energy and low-grade ambient heat via a photoinduced crystal-to-liquid transition

Abstract

Ambient heat, slightly above or at room temperature, is a ubiquitous and inexhaustible energy source that has typically been ignored due to difficulties in its utilization. Recent evidence suggests that a class of azobenzene (Azo) photoswitches featuring a reversible photoinduced crystal-to-liquid transition could co-harvest photon energy and ambient heat. Thus, a new horizon has been opened for recovering and regenerating low-grade ambient heat. Here, a series of unilateral para-functionalized photoinduced liquefiable Azo derivatives is presented that can co-harvest and convert photon energy and ambient heat into chemical bond energy and latent heat in molecules and eventually release them in the form of high-temperature utilizable heat. A straightforward crystalline-to-liquid phase transition achieved with ultraviolet light irradiation (365 nm) is enabled by appending a halogen/alkoxy group on the para-position of the Azo photoswitches, and the release of thermal energy is triggered by short-wavelength visible-light irradiation (420 nm). The phase transition properties of the trans- and cis-isomers and the energy density, storage lifetime and heat release performance of the cis-liquid are investigated with differential scanning calorimetry, ultraviolet-visible absorption spectroscopy, and an infrared (IR) thermal camera. The experimental results indicate a high energy density of 335 J/g, a long lifetime of 5 d and a heat release of up to 6.3 °C due to the coupled photochemical-thermophysical mechanism. This work presents a new model for utilizing renewable energy, i.e., the photoinduced conversion of ambient thermal energy.

Optically Controlled Solid-to-Liquid Phase Transition Materials Based on Azo Compounds

“…On the contrary, the advantage of photoliquefiable azo compounds that can simultaneously store photon energy and ambient latent heat should be further developed. As mentioned in section , the combination of azo compounds and conventional PCMs could improve the energy storage capacity and expand the latent heat storage to a low-temperature range (Figure c). ,,, In addition, some azo compounds complete the photochemical phase transition above room temperature, to utilize ambient latent heat in a high-temperature range (Figure a) . In that way, solar energy and ambient thermal energy could be stored comprehensively, adapting to various working temperature ranges.…”

“…Furthermore, Dong et al controlled the photoinduced crystallization by using a cover board, demonstrating a selective energy release process with a larger ΔT (Figure 15c). 83 Because STFs release the stored energy in the form of heat, they can be mainly used in temperature regulating equipment such as heaters. At present, the directly usable energy in human life and industrial production is still dominated by electric energy, so it is meaningful to further convert the heat energy of STFs into electric energy.…”

“…The electron-donating and -withdrawing properties will also influence the dipole moments of the azo compounds as well as their photoinduced phase transition ability. 73,83 Because research on azo-OPTMs has been conducted for more than 10 years, there are already many results in terms of their molecular designs and phase transition mechanisms. However, there is still no clear theory to fully reveal the relationship between the molecular structure and phase transition process, which still requires researchers to carry out further experiments and dynamic simulations.…”

“…Selective energy release: (c) schematic and panel d and e relative to panels a and b, respectively. Adapted with permission under a Creative Commons CC BY 4.0 License from ref . Copyright 2022 OAE Publishing Inc.…”

“…In this way, the operating temperature range of STFs can be widened, dealing with a low-temperature environment (Figure c). Furthermore, Dong et al controlled the photoinduced crystallization by using a cover board, demonstrating a selective energy release process with a larger Δ T (Figure c) …”