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

Abstract

Solar thermal fuels (STFs) harvest and store solar energy in a closed cycle system through conformational change of molecules and can release the energy in the form of heat on demand. With the aim of developing tunable and optimized STFs for solid-state applications, we designed three azobenzene derivatives functionalized with bulky aromatic groups (phenyl, biphenyl, and tert-butyl phenyl groups). In contrast to pristine azobenzene, which crystallizes and makes nonuniform films, the bulky azobenzene derivatives formed uniform amorphous films that can be charged and discharged with light and heat for many cycles. Thermal stability of the films, a critical metric for thermally triggerable STFs, was greatly increased by the bulky functionalization (up to 180°C), and we were able to achieve record high energy density of 135 J/ g for solid-state STFs, over a 30% improvement compared to previous solid-state reports. Furthermore, the chargeability in the solid state was improved, up to 80% charged from 40% charged in previous solid-state reports. Our results point toward molecular engineering as an effective method to increase energy storage in STFs, improve chargeability, and improve the thermal stability of the thin film. KEYWORDS: solar thermal fuels heat storage, molecular thin films, solid-state applications, structural design, molecular engineering, photoswitching ■ INTRODUCTIONDespite the vast abundance of solar radiation, efficient conversion, storage, and distribution of this resource remains a challenge. One approach that uses the same material to both convert and store the sun's energy is the use of photoactive molecules. These molecules convert solar energy into strained or rearranged chemical bonds, with the amount of energy stored, ΔH, as the difference in energy between the ground and metastable states. Upon reversion back to the ground state, the energy is released in the form of heat with the molecules back in their ground state ready to be charged again. These materials for solar energy harvesting, referred to as solar thermal fuels (STF), operate in a closed cycle and ideally possess high energy density and cyclability with no degradation or emissions and easy distribution as "heat on demand".Previous work on candidate solar thermal fuels has shown both promise and many challenges in optimizing the required properties. For example, norbornadiene showed great potential with high energy density (89 kJ/mol). Although cyclability has been the biggest challenge, recent research by molecular modification of norbornedine using aryl substituents showed improvement of cyclability.1−3 Recent work showed that (Fulvalene) tetracarbonyl-diruthenium showed reasonable gravimetric energy density (30.6 Wh/kg) while showing high cyclability, but due to the use of expensive ruthenium, its potential is limited.4,5 Azobenzene possesses high cyclability but has been hindered by low energy density. 6 Recently, several templating methods of the solar thermal fuel molecule have gained attention and have been invest...

High Energy Density in Azobenzene-based Materials for Photo-Thermal Batteries via Controlled Polymer Architecture and Polymer-Solvent Interactions

“…S25 ). From all of these experiments, we concluded that the observed exotherm in Z -AzoPMA 3 is associated with discharging, is exclusively from the isomerization of the Z -isomer to the E -isomer, and is consistent with observations made by others 2 , 3 , 22 – 24 . We thus used AzoPMA 3 for the remainder of our studies.…”

Liquid Norbornadiene Photoswitches for Solar Energy Storage

“…To put this value into context it is interesting to compare it to other valid MOST candidates, based for example on azobenzene derivatives. Promising solid MOST materials based on neat azobenzenes have been reported, with energy density up to about 100 kJ kg −1 (and 88 kJ mol −1 ); liquid azobenzene derivatives have also been made, reaching a gravimetric energy density of 115 kJ kg −1 . The energy density of the reported system a translates to a possible adiabatic temperature rise of 384 °C, underlining the potential of this class of compounds for MOST applications.…”

Intermolecular London Dispersion Interactions of Azobenzene Switches for Tuning Molecular Solar Thermal Energy Storage Systems

“…While these ones are mostly based on the norbornadiene‐quadricyclane system, other molecular entities have also been proposed . Herein, the azobenzene represents a highly potent candidate as there is longstanding experience in studying this system as a dye as well as a molecular switch . The storage energy of a given MOST system is based on the energy differrence between two states.…”

“…[3] Herein, the azobenzene represents a highly potent candidate as there is longstanding experience in studying this system as a dye as well as a molecular switch. [7,8] The storage energy of a given MOST system is based on the energy differrence between two states. In case of the azobenzene, this is the (E)-isomer as the lower energy state and the (Z)-isomer as the metastable high-energy state ( Figure 1).…”