Solar–Thermal Fuels and the Role of Carbon Nanomaterials: A Perspective with Emphasis on the Azobenzene System

Molecular switches have long been investigated aiming at developing sustainable and renewable energy-based technologies. Among molecular photoswitches, azobenzene (AB) has been the quintessential template for solar–thermal fuels (STFs), acting as "molecular batteries", storing and discharging energy by converting between its trans- and cis-isomers upon absorption of UV light. Nevertheless, key properties for device performance such as stored energy, energy density, stability of the "charged" metastable isomer, or the control of energy release are highly influenced by a mediating interface. This has motivated the study of metal and carbon nanomaterials as templates in hybrid STFs. The latter have been shown to be especially promising owing to their unique electronic properties, topological tailoring, and abundant sourcing. This review discusses recent advances that explore enhancements on several molecular photoswitch systems, with a special emphasis on the AB system, associated with changes in the molecular substituents and the development of hybrid systems, involving mainly carbon nanotemplates, such as reduced graphene oxide, graphene, carbon nanotubes, and fullerenes. The energy density, τ1/2, grafting density, and charge transfer of the AB and the hybrid adsorbed AB system on carbon nanomaterials are discussed. Key facets of the design of hybrid STF devices are discussed, along with a perspective on the aspects that merit further investigation toward the development of efficient STFs.