Solid‐State Solar Thermal Fuels for Heat Release Applications

Abstract

56 Wh kg −1 ) employing ruthenium with higher cyclability have been developed, [5][6][7] but remain prohibitively expensive for wide adoption. With the availability of accurate computational tools, new approaches have leveraged carbon nanostructures and insights into the steric interaction of STFs to increase energy density employing highly cyclable and modest energy density (60-70 Wh kg −1 ) azobenzene derivatives. [ 8,9 ] While demonstrating a per-molecule increase in energy density via templating, [ 10 ] these approaches require complex multistep reactions, have low yields, and the resulting material has low solubility in most organic solvents (<1 g L −1 ). More recently, it was possible to develop liquid azobenzene fuels at room temperature by attaching bulky ligands to the molecule, [ 11 ] and with several computational works detailing the possibility of increasing its energy density through functionalization of the benzene rings, [ 12 ] this platform holds much promise for future STF developments. With such rapid progress in STF materials, it is perhaps surprising that the solid-state platform and related applications have remained largely unexplored, with only recent studies on semi-solid photoliquefi able ionic crystals reaching energy densities of 35 Wh kg −1 . [ 13 ] Transitioning fully to the solid-state offers the possibility of integrating STF materials into a multitude of existing solid-state devices such as coatings for deicing, or novel applications such as solar blankets and other consumer oriented heating equipment.We took the view that if properly engineered on the molecular level, STF materials could be controllably tailored within the solid-state, and that until now, there has not been an efficient method to accomplish this. For one, the most recent STF reports have relied on carbon scaffolds [ 10,14 ] that simultaneously increase synthesis complexity, cannot be deposited into uniform fi lms, contribute to the optical density without resulting in photocharging, and introduce uncontrollable morphological effects that may limit charging and reversible switching in the solid-state. [ 15 ] Similarly, single-molecule thin fi lms do not make homogenous layers, can often result in crystallization, and melt at low temperatures (≈70 °C for azobenzene) thus limiting their utility in the solid-state. Fortunately, a wealth of literature exists on azobenzene-based materials in solid-state applications for microswitches, microactuators, and sensors. [16][17][18][19][20][21] We postulated that the ideal material class to form solid-state STF coatings would need to (1) form smooth fi lms with controllable thickness, (2) be resilient at high temperatures, (3) preserve the heat release properties of Closed cycle systems offer an opportunity for solar energy harvesting and storage all within the same material. Photon energy is stored within the chemical conformations of molecules and is retrieved by a triggered release in the form of heat. Until now, such solar thermal fuels (STFs) have been largely unavailable...

Molecularly Engineered Azobenzene Derivatives for High Energy Density Solid-State Solar Thermal Fuels

“…31 The enthalpy difference between cis and trans isomers was determined using DSC in the solid-state (Figure 2e), which showed a heat release (4 measurements) of 134 ± 6 J/g or 77 ± 3 kJ/mol for compound 1, significantly larger than that of azobenzene (∼49 kJ/mol) and over a 30% increase from the polymer azobenzene. 6,10 In contrast to azobenzene and azobenzene template structures that typically yield one exothermic peak for the cis to trans release of stored heat, 9,10,32,33 compound 1 showed two peaks, which reveals an important aspect of processing small-molecule films as STF materials, as will be discussed below.…”

“…The increase in percent charged and increase in energy stored per molecule is important because, although the molecular weight of compound 1 is 3 times that of azobenzene, the gravimetric energy density is measured to be ∼134 J/g (37 Wh/kg), which is comparable to that of the azobenzene molecule and higher than that of the polymer template azobenzene by 30%. 10,26 The prevention of crystallization of compound 1 allows for the fabrication of stable, amorphous thin films. Crystallization (as in the case of pure azobenzene) presents a challenge for solidstate applications because the steric hindrance between the molecules in the crystal prevents the molecule from switching between the trans and the cis state.…”

“…Of these, the polymer approach has thus far proved most successful, allowing robust solid-state STF 10 and controlled thickness. 13 Solid-state STF opens a window to a range of applications by integrating STF materials to existing solid-state devices such as coatings for deicing, solar blankets and functional fibers, or implementation in new novel solid state energy devices for consumer oriented heating equipment.…”

Light‐Switchable Azobenzene‐Containing Macromolecules: From UV to Near Infrared

“…The cis isomer can release the stored energy as heat. Recently, Grossman and co‐workers demonstrated the use of an azopolymer as solar energy storage . The use of azopolymers provides the opportunity to modify the structures of the monomers and polymer backbone to enhance the stored energy density, improve the optical chargeability and photostationary state, and collect photons across a greater portion of the solar spectrum.…”

Photon energy storage materials with high energy densities based on diacetylene–azobenzene derivatives

“…Lower photon energy storage in polymers compared to monomers has also been recently reported by Zhitomirsky, et al for azobenzene-functionalized PMMA structures. 45 We assume that the polymer aggregation in the dispersion hinders light absorption by the materials, limiting the conversion of azobenzene side chains. However, the trend of DH per mole (kJ mol…”