High Throughput Virtual Screening of 200 Billion Molecular Solar Heat Battery Candidates

The dihydroazulene/vinylheptafulvene (DHA/VHF) thermocouple is a promising candidate for thermal heat batteries that absorb and store solar energy as chemical energy without the need for insulation. However, in order to be viable the energy storage capacity and stability of the high energy form (the free energy barrier to the back reaction) must be increased significantly. We use semiempirical quantum chemical methods, machine learning, genetic algorithms, and density functional theory to virtually screen roughly 200 billion substituted DHA molecules to identify promising candidates for further study. We identify three molecules with predicted energy densities of (0.34-0.36 kJ/g), which is significantly larger than the 0.14 kJ/g computed for the parent system. The free energy barriers to the back reaction are between 6.8 and 7.7 kJ/mol higher than the parent compound, which should correspond to half-lives of days - sufficiently long for many practical applications.