Trimodal thermal energy storage material for renewable energy applications

The global aim to move away from fossil fuels requires efficient, inexpensive and sustainable energy storage to fully use renewable energy sources. Thermal energy storage materials1,2 in combination with a Carnot battery3–5 could revolutionize the energy storage sector. However, a lack of stable, inexpensive and energy-dense thermal energy storage materials impedes the advancement of this technology. Here we report the first, to our knowledge, ‘trimodal’ material that synergistically stores large amounts of thermal energy by integrating three distinct energy storage modes—latent, thermochemical and sensible. The eutectic mixture of boric and succinic acids undergoes a transition at around 150 °C, with a record high reversible thermal energy uptake of 394 ± 5% J g−1. We show that the transition involves melting of the boric acid component, which simultaneously undergoes dehydration into metaboric acid and water that dissolve into the liquid. Being retained in the liquid state allows the metaboric acid to readily rehydrate to re-form boric acid on cooling. Thermal stability is demonstrated over 1,000 heating–cooling cycles. The material is very low cost, environmentally friendly and sustainable. This combination of a solid–liquid phase transition and a chemical reaction demonstrated here opens new pathways in the development of high energy capacity materials.