The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. One approach is the development of energy storage systems based on molecular photoswitches, so-called molecular solar thermal energy storage (MOST). Here we present a novel norbornadiene derivative for this purpose, with a good solar spectral match, high robustness and an energy density of 0.4 MJ kg⁻¹. By the use of heterogeneous catalyst cobalt phthalocyanine on a carbon support, we demonstrate a record high macroscopic heat release in a flow system using a fixed bed catalytic reactor, leading to a temperature increase of up to 63.4 °C (83.2 °C measured temperature). Successful outdoor testing shows proof of concept and illustrates that future implementation is feasible. The mechanism of the catalytic back reaction is modelled using density functional theory (DFT) calculations rationalizing the experimental observations.

Molecules capable of reversible storage of solar energy have recently attracted increasing interest, and are often referred to as molecular solar thermal energy storage (MOST) systems. Azobenzene derivatives have great potential as an active MOST candidate. However, an operating lab scale experiment including solar energy capture/storage and release has still not been demonstrated. In the present work, a liquid azobenzene derivative is tested comprehensively for this purpose. The system features several attractive properties, such as a long energy storage half-life (40 h) at room temperature, as well as an excellent robustness demonstrated by optically charging and discharging the molecule over 203 cycles without any sign of degradation (total operation time of 23 h). Successful measurements of solar energy storage under simulated sunlight in a microfluidic chip device have been achieved. The identification of two heterogeneous catalyst systems during testing enabled the construction of a fixed bed flow reactor demonstrating catalyzed back-conversion from cis to trans azobenzene at room temperature under flow conditions. The working mechanism of the more suitable catalytic candidate was rationalized by detailed density functional theory (DFT) calculations. Thus, this work provides detailed insights into the azobenzene based MOST candidate and identifies where the system has to be improved for future solar energy storage applications.

Molecular switches based on E/Z photoisomerization have been used in different contexts in order to control a variety of processes in different systems, from peptide conformation control to molecular data storage devices, from catalysis to smart materials. The syntheses, properties and applications of several types of E/Z photochemical switches are presented with special attention paid to azobenzenes, overcrowded alkenes and switches based on the protonated Schiff base chromophore of rhodopsins.