Confinement in metal-organic frameworks as a route to harnessing liquid barocalorics in the solid-state

TL;DR

The study demonstrates that solid–liquid barocaloric transitions of stearic acid can be preserved when confined inside the nanopores of a functionalized MOF (MIL-101(Cr)-NH2), enabling large, reversible BC effects in a solid-state material. Through synthesis, diffraction, SEM, porosimetry, and high-pressure calorimetry, the authors show confinement depresses the SA transition temperature and that reversible entropy and temperature changes of substantial magnitude are achievable at practical pressures (≈100–242 MPa). The work highlights active control of BC responses via MOF–adsorbate interactions and paves the way for tunable, leak-free solid-state BC devices using a broad range of solid–liquid PCM/MOF combinations. Overall, this approach combines the high latent heat of molten phases with the containment and recyclability of solids, offering a promising route for next-generation, environmentally friendly cooling technologies.

Abstract

Barocaloric (BC) effects at liquid-vapor transitions in hydrofluorocarbons drive most commercial technologies used for heating and cooling in the heating, ventilation and air-conditioning sector. However, these fluids suffer from huge global warming potential and alternative gases are less efficient, toxic or flammable. Solid-solid and solid-liquid BC materials have zero global warming potential and could even improve on current device efficiencies. Whilst solid-liquid BCs typically outperform solid-solid BCs, the latter are advantageous as they avoid leaks and present easier handling and recyclability thus facilitating waste management. Here we confine the solid-liquid BC stearic acid inside the nanopores of a functionalised metal-organic framework (MOF) and demonstrate that the colossal BC properties are retained in a solid-state material. Moreover, the enhanced interactions between the pore surface and the BC material allow a level of active control over the thermal response, as opposed to passive encapsulation. Our results open novel avenues to exploit and tune colossal BC effects in a wide range of combinations of solid-liquid BC materials embedded within functionalized MOFs, without the associated engineering drawbacks.

Figures (6)

• Figure 1: (a) Synchrotron diffraction data on MIL along with a simulated diffraction pattern based on the expected MIL crystal structure. The MIL crystal structure is shown in the inset, with the largest pore sizes indicated; (b) Temperature dependent synchrotron diffraction data; (c) Refined volume from data in (b) of the MIL phase; (d-e) SEM images of (d) $x=0$ and (e) $x=40\%$ of SA@MIL.
• Figure 2: (a) Ambient pressure calorimetric signal as a function of temperature for different SA concentration. (b) Transition temperature as a function of the SA concentration. Solid and empty symbols correspond to peak maxima and onsets, respectively. Lines are fits to peak maxima. (c) Transition entropy change per unit of mass of SA@MIL, as a function of SA concentration.
• Figure 3: (a-e) High-pressure calorimetric signals measured at different applied pressures and (f-j) as derived respective temperature-pressure phase diagrams in SA@MIL samples with different SA concentration: (a,f) $x=20\%$, (b,g) $x=40\%$, (c,h) $x=60\%$, (d,i) $x=80\%$, (e,j) $x=100\%$. Solid and empty symbols correspond to peak maxima and onsets, respectively. Lines are fits to peak maxima.
• Figure 4: (a-e) Reversible isothermal entropy changes and (f-j) and reverstible adiabatic temperature changes upon pressure changes for different SA concentrations. Color legends in panels (f,g,h,i,j) also hold for panels (a,b,c,d,e), respectively.
• Figure 5: (a) Maximum reversible isothermal entropy changes and (b) maximum reversible adiabatic temperature changes displayed as a function of SA concentration and pressure changes.