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

Abstract

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.

Figure 1

Representation of the narrow-pore np-phase (left) and large-pore lp-phase (right) of MIL-53(Al) viewed along the axis of the unidimensional channels under compression and decompression with CO₂. Note: CO₂ molecules have been randomly allocated for visualization purposes.

Figure 2

VT-DSC curve on heating ramps (10 K min^–1) under different CO₂ isobaric conditions (p = [1.5–5] bar). Left inset: pressure dependence of the transition temperature. Right inset: pressure dependence of the latent heat.

Figure 3

Pressure–temperature phase diagram of MIL-53(Al) under CO₂ atmosphere, where the solid black line represents the reported simulated profile calculated by adsorption isotherms data, the red points indicate the reported experimental points obtained by adsorption isotherms, both taken from ref (26), and the blue points represent the experimental data obtained by VT-DSC in the present work.

Figure 4

Pressure and heat flow signals obtained during CO₂ adsorption on MIL-53(Al) at 300 K using a point-by-point procedure of gas introduction.

Figure 5

Heat flow dQ/dp on cycles of applying (0 → p) and removing (p → 0) CO₂-pressure at different temperatures (from 298 to 333 K) at the same rate of dp/dt ∼ 1.6 bar min^–1. Note: curves have been vertically shifted for facilitating visualization.

Figure 6

(a) Variation of transition pressure, p_t, of the breathing transition as a function of operating temperature. (b) Variation of ΔH as a function of operating temperature. (c) Variation of ΔS as a function of operating temperature. Note: data represented from the pressurization curves.

Figure 7

(a) Ashby plot of ΔS vs p for MIL-53(Al) and different refrigerants.^−,,,, (b) Comparison of the operating temperature range (T_span) of the best refrigerants selected from (a). Note: In panel a, GAS = gas refrigerants, HOIMs = hybrid organic–inorganic materials, OPC = organic plastic crystals, SCO = spin crossover materials, POLY = polymers, AIS = ammonium inorganic salts, ISC = ionic superconductors. In panel b, the pink striped area indicates the T_span below RT for MIL-53(Al) estimated from reported BET isotherms.

Figure 8

Ideal Brayton cycle that illustrates a possible cooling/heating cycle based on the MIL-53(Al) breathing-caloric effect. The cycle consists of four stages: (1 → 2) adiabatic pressurization of MIL-53, which in turn increases its temperature, (2 → 3) heat release (Q^–) from MIL-53 under isobaric conditions, (3 → 4) adiabatic depressurization of MIL-53, which further decreases its temperature, and (4 → 1) heat from the surroundings (Q⁺) is absorbed by MIL-53 under isobaric conditions. Note: the heat released in stage 2 → 3 can be used for heating applications or just discarded as residual heat, while the heat absorbed in stage 4 → 1 is useful to cool a fridge chamber and/or a room in the case of an air conditioning system.