High entropy sulfonium-based organic ionic plastic crystals for sustainable cooling

Abstract

Barocaloric (BC) materials, being those in which a thermal change is driven by the application and removal of hydrostatic pressure, offer an energy efficient, solid-state alternative to ubiquitous but problematic volatile hydrofluorocarbon refrigerants. However, finding BC materials with high entropy transitions occurring within the required sub-ambient temperature regions remains a challenge. Here, we report a new family of barocaloric organic ionic plastic crystals (OIPCs) based on seldom-explored sulfonium cations that demonstrate solid–solid phase transitions exhibiting strikingly high entropy changes, up to 375 J kg⁻¹ K⁻¹. These transitions are pressure-sensitive (up to 14.5 K kbar⁻¹) and occur at temperatures suited to refrigeration and freezing technologies (−20 °C to 4 °C), with low transition hysteresis (<1 °C for [S₁₁₁][FSI]). This combination of properties leads to excellent performance over a range of BC metrics, with pressure-normalised refrigeration capacities among the highest to-date and as high as 2.9 J kg⁻¹ bar⁻¹. For the first time, we also demonstrate the pressure cyclability of BC OIPCs, finding [S₁₁₁][FSI] retains its exceptional entropy change and low hysteresis over 100 DTA pressure cycles. The nature of the molecular structure and dynamics introduced at the transitions, which underpins their large entropy changes, is investigated using solid-state nuclear magnetic resonance spectroscopy and positron annihilation lifetime spectroscopy, the latter being a novel technique in the BC context. We elucidate structural features of the constituent ions that benefit the BC performance of OIPCs, demonstrating their tunability and as-yet untapped potential for application in sustainable cooling technologies.