In this work, “breathing-caloric” effect is introduced as a new term to define very large thermal changes that arise from the combination of structural changes and gas adsorption processes occurring during breathing transitions. In regard to cooling and heating applications, this innovative caloric effect appears under very low working pressures and in a wide operating temperature range. This phenomenon, whose origin is analyzed in depth, is observed and reported here for the first time in the porous hybrid organic–inorganic MIL-53(Al) material. This MOF compound exhibits colossal thermal changes of ΔS ∼ 311 J K^–1 kg^–1 and ΔH ∼ 93 kJ kg^–1 at room temperature (298 K) and under only 16 bar, pressure which is similar to that of common gas refrigerants at the same operating temperature (for instance, p(CO₂) ∼ 64 bar and p(R134a) ∼ 6 bar) and noticeably lower than p > 1000 bar of most solid barocaloric materials. Furthermore, MIL-53(Al) can operate in a very wide temperature range from 333 K down to 254 K, matching the operating requirements of most HVAC systems. Therefore, these findings offer new eco-friendly alternatives to the current refrigeration systems that can be easily adapted to existing technologies and open the door to the innovation of future cooling systems yet to be developed.

Organometallic-sandwich salts are well-known materials that undergo order-disorder phase transitions, leading to a high-temperature phase characterized by the total or partial disorder of ionic species. Their potential for barocaloric applications has not previously been explored. Here, we focus on two salts derived from metallocenes with the formula [Cp₂M][PF₆] where Cp: cyclopentadienyl anion (C₅H₅)^- and M: Fe³⁺ or Co³⁺. These molecular salts exhibit two solid-solid phase transitions, shifting from a well-ordered crystalline phase at low temperatures to an orientationally disordered phase in the case of the Fe-compound, and a partially disordered phase in the case of the Co-compound above room temperature. We find a significant entropy change (∼40 J K^-1 kg^-1) and a moderate volume change (∼2%) associated with these phase transitions. Additionally, we observe that these transitions are highly sensitive to external applied pressure, leading to substantial barocaloric effects (exceeding 20 K kbar^-1). Very interestingly, we obtain substantial values for reversible adiabatic temperature change (ΔT_rev > 10 K) under an applied pressure as low as 1 kbar, comparable to those observed in the most promising barocaloric materials. We prove that the unique structure and chemical bonding of these sandwich organometallic cations are responsible for their interesting and unusual barocaloric response. These findings position the metallocenium family as promising candidates for eco-friendly solid-state refrigeration technologies.

The innovative [DBA][BF4] thermomaterial exhibits advanced thermal properties for multipurpose active barocaloric refrigeration and passive cold-storage of interest for commercial food preservation.

Today, ≈20% of the electric consumption is devoted to refrigeration; while, ≈50% of the final energy is dedicated to heating applications. In this scenario, many cooling devices and heat‐pumps are transitioning toward the use of CO₂ as an eco‐friendly refrigerant, favoring carbon circular economy. Nevertheless, CO₂ still has some limitations, such as large operating pressures (70–150 bar) and a critical point at 31 °C, which compromises efficiency and increases technological complexity. Very recently, an innovative breathing‐caloric mechanism in the MIL‐53(Al) compound is reported, which implies gas adsorption under CO₂ pressurization boosted by structural transitions and which overcomes the limitations of stand‐alone CO₂. Here, the breathing‐caloric‐like effects of MOF‐508b are reported, surpassing by 40% those of MIL‐53(Al). Moreover, the first thermometry device operating at room temperature and under the application of only 26 bar of CO₂ is presented. Under those conditions, this material presents values of ΔT ≈ 30 K, reaching heating temperatures of 56 °C and cooling temperatures of −10 °C, which are already useful for space heating, air‐conditioning, food refrigeration, and freezing applications.

In this work, we design, build, and test one of the very first barocaloric devices. The here presented device can recover the energy generated by an individual’s footstep and transform it into barocaloric heating and/or cooling. Accordingly, we present an innovative device that can provide eco-friendly and gas-free heating/cooling. Moreover, we test the device by measuring a new barocaloric organic polymer that exhibits a large adiabatic temperature change of ~2.9 K under the application of 380 bar. These results pave the way towards novel and more advanced barocaloric technologies and provide a simple and low-cost device to explore new barocaloric materials.