Hybrid ionic plastic crystals in the race for enhanced low-pressure barocaloric materials

Abstract

In this work, we introduce a new family of barocaloric hybrid organic-inorganic compounds with colossal barocaloric effects. The here reported hybrid materials, [(CH3)3(CH2Cl)N]FeCl4 and [(CH3)3S]FeCl4, exhibit a molecular structure composed by discrete inorganic anions and organic cations with weak elestrostatic interactions. Our calorimetric studies reveal colossal barocaloric effects of similar magnitude than organic plastic crystals (ΔS > 100 J K-1 kg-1) near room temperature and under smaller pressures (p ≤ 1000 bar), which leads to higher barocaloric strengths. Furthermore, these materials exhibit densities similar to barocaloric hybrid perovskites enhancing the volumetric barocaloric effects (ΔS ~ 200 J K-1 l-1), which could provide more compact cooling devices. Therefore, the colossal values of the mass and volumetric barocaloric effects and large barocaloric strength, in addition to the low working pressure and near-room-temperature operation, offer a new family of compounds to further explore in the search for improved barocaloric materials.

Discovery of Colossal Breathing-Caloric Effect under Low Applied Pressure in the Hybrid Organic–Inorganic MIL-53(Al) Material

“…Additionally, T span of MIL-53(Al) is also superior to that of the best reported barocalorics 5 − 11 and perfectly matches with that of low-temperature CO 2 systems. 34 − 37 46 Therefore, MIL-53(Al) would be an ideal complementary refrigerant that would operate above 298 K, requiring lower pressure than stand-alone CO 2 .…”

“…Therefore, the required pressure is in the range of commercial gas refrigerants for vapor-compression technologies (for instance, p (CO 2 ) ∼ 64 bar and p (R134a) ∼ 6 bar) 46 and is much lower than in the case of traditional solid barocalorics. 5 − 11 , 15 , 16 , 45 In view of these results, MIL-53(Al) shows a temperature span of, at least, 35 K for p = 16 bar, which widely exceeds the span of most barocaloric materials. 5 − 11 , 15 , 16 , 45 …”

“… 5 − 11 , 15 , 16 , 45 In view of these results, MIL-53(Al) shows a temperature span of, at least, 35 K for p = 16 bar, which widely exceeds the span of most barocaloric materials. 5 − 11 , 15 , 16 , 45 …”

“… 5 − 11 , 15 , 16 , 45 Even more remarkably, the breathing-caloric effect is fully reversible under the application of very low pressures ( p = 16 bar,) in contrast to the p > 1000 bar required for most barocalorics. 5 − 11 Accordingly, the caloric strength of the material, defined as the isothermal entropy change per unit of pressure, Δ S /Δ p ∼ 19.4 × 10 3 J K –1 kg –1 kbar –1 , is the largest strength reported to date (see below).…”

“…In this pressing scenario, and looking for alternatives, solid-state materials that can present pressure-induced phase transitions are arising as a promising alternative to refrigerant gases. These solid materials, known as barocaloric compounds, can exhibit large thermal changes (isothermal entropy changes Δ S or adiabatic temperature changes Δ T ) when undergoing a solid to solid phase transition induced by the application and removal of isostatic pressure, in a similar way to refrigerant gases. − In general, barocaloric materials can offer many advantages over gas refrigerants: they cannot escape to the atmosphere, they are easier to recover and reuse in case of system breakage, they can be transported in nonpressurized vessels, and they can lead toward more compact systems, among others.…”