Reversible Polymerization Chemistry for Compact Thermochemical Energy Storage Systems

Thermal energy storage (TES) can be an effective alternative to electrochemical batteries in applications where the end usage is primarily heat¹. Within TES, thermochemical energy storage (TCS) based on reversible chemical reactions have been demonstrated for high-temperature applications via gas-gas or solid-gas reactions based on ammonia, sulfuric acid, limestone, etc². However, the development of these inorganic systems is limited by material degradation with cycling, hysteresis losses, and complex equipment and operation. Recent studies have explored organic reactions that operate in a homogeneous phase for low to medium temperature TCS applications³. With significant advances in chemically recyclable polymers⁴, we investigate here the polymerization/depolymerization chemistry of ε-caprolactone/polycaprolactone as a viable candidate for TCS. This chemistry is advantageous from an applications perspective due to a simple operation as a closed system and a broad range of charge/discharge temperatures suitable for water heating (100 – 200 °C). Preliminary studies confirm high conversion in the forward direction (polymerization) and selective formation of the monomer in the reverse direction. We find the heat of reaction obtained from DSC experiments (∼ 140 J/g) to be consistent with prior literature values⁵. We discuss ongoing experiments to enhance the monomer yield during depolymerization and to quantify heat of polymerization (storage capacity) using a laboratory-scale reactor. This work advances the usage of organic compounds for TES applications and could form the basis for further identification of similar chemistries.<br/><br/><i>1. Odukomaiya, A. et al. Addressing energy storage needs at lower cost via on-site thermal energy storage in buildings. Energy and Environmental Science vol. 14 5315–5329 Preprint at https://doi.org/10.1039/d1ee01992a (2021).</i><br/><i>2. Prieto, C., Cooper, P., Fernández, A. I. & Cabeza, L. F. Review of technology: Thermochemical energy storage for concentrated solar power plants. Renewable and Sustainable Energy Reviews vol. 60 909–929 Preprint at https://doi.org/10.1016/j.rser.2015.12.364 (2016).</i><br/><i>3. Spotte-Smith, E. W. C., Yu, P., Blau, S. M., Prasher, R. S. & Jain, A. Aqueous Diels–Alder reactions for thermochemical storage and heat transfer fluids identified using density functional theory. J Comput Chem <b>41</b>, 2137–2150 (2020).</i><br/><i>4. Coates, G. W. & Getzler, Y. D. Y. L. Chemical recycling to monomer for an ideal, circular polymer economy. Nature Reviews Materials vol. 5 501–516 Preprint at https://doi.org/10.1038/s41578-020-0190-4 (2020).</i><br/><i>5. Lebedev, B. V et al. The thermodynamics of ε-caprolactone, its polymer and of ε-caprolactone polymerization in the 0-350 K range. Polymer Science U.S.S.R vol. 20.</i>