Barocaloric Design Principles in Asymmetric Dialkylammonium Salts

Conventional cooling cycles rely on the vapor-compression of hydrofluorocarbon (HFC) and hydrofluoroolefin (HFO) refrigerants, which can pose serious environmental and safety hazards upon leakage into the atmosphere. Solid-state barocaloric materials—materials which undergo thermal changes upon application of hydrostatic pressure—offer a promising path towards more sustainable cooling technologies. In recent years, materials that undergo hydrocarbon chain disordering transitions (so-called 'chain melting' transitions) have emerged as especially promising barocaloric candidates, owing to their large entropy changes (>200 J/kg/K) accessible at relatively low driving pressures (<200 bar). However, the phase space for chain-melting materials remains somewhat narrow, making it difficult to investigate structure-property relationships and design principles for this family of barocaloric materials.

Here, we report asymmetric dialkylammonium salts as a tunable platform for investigating barocaloric structure-property relationships in chain-melting materials. Through a combination of calorimetric and structural studies, we investigate the effect of systematic alkyl chain truncation on chain-melting behavior. In doing so, we identify a subset of asymmetric dialkylammonium salts that retain competitive gravimetric entropy changes while benefiting from reduced hysteresis, which has implications for achieving greater cooling efficiencies and reducing driving pressures. Ultimately, this work expands the existing library of chain-melting materials for barocaloric applications and offers insights towards their rational and controllable manipulation.