Low pressure reversibly driving colossal barocaloric effect in two-dimensional vdW alkylammonium halides

Abstract

Plastic crystals as barocaloric materials exhibit the large entropy change rivalling freon, however, the limited pressure-sensitivity and large hysteresis of phase transition hinder the colossal barocaloric effect accomplished reversibly at low pressure. Here we report reversible colossal barocaloric effect at low pressure in two-dimensional van-der-Waals alkylammonium halides. Via introducing long carbon chains in ammonium halide plastic crystals, two-dimensional structure forms in (CH₃-(CH₂)_n-1)₂NH₂X (X: halogen element) with weak interlayer van-der-Waals force, which dictates interlayer expansion as large as 13% and consequently volume change as much as 12% during phase transition. Such anisotropic expansion provides sufficient space for carbon chains to undergo dramatic conformation disordering, which induces colossal entropy change with large pressure-sensitivity and small hysteresis. The record reversible colossal barocaloric effect with entropy change ΔS_r ~ 400 J kg^-1 K^-1 at 0.08 GPa and adiabatic temperature change ΔT_r ~ 11 K at 0.1 GPa highlights the design of novel barocaloric materials by engineering the dimensionality of plastic crystals.

Fig. 1. Crystalline structure and thermal properties of phase transition for (CH₃–(CH₂)₉)₂NH₂Cl.

a Crystalline structure of (CH₃–(CH₂)₉)₂NH₂Cl at 300 K, where the unit cell is marked by black frame. b Molecular structure of (CH₃–(CH₂)₉)₂NH₂Cl. c N–H…Cl hydrogen bond interaction in (CH₃–(CH₂)₉)₂NH₂Cl, where partial carbon and hydrogen atoms are omitted. d DSC measurement of (CH₃–(CH₂)₉)₂NH₂Cl at atmosphere pressure with temperature ramping rate 0.1 K min⁻¹. e Temperature-dependence of entropy change across phase transition calculated from (d).

Fig. 2. Barocaloric performance of (CH₃–(CH₂)₉)₂NH₂Cl obtained by the quasi-direct method.

a Heat flow curves under variable pressure measured at a temperature rate 1 K/min. b Pressure-dependent phase transition temperature from (a). c Isothermal entropy change ΔS at variable pressure from the pressure-variable entropy curves (Supplementary Fig. S5). d Reversible entropy change ΔS_r by overlapping of pressurization and depressurization from (c). e Adiabatic temperature change ΔT_ad from pressure-variable entropy curves (Supplementary Fig. S7). f Reversible adiabatic temperature change ΔT_r from heating entropy curve at atmosphere pressure and cooling entropy curves under applied pressure.

Fig. 3. The comparison of reversible barocaloric entropy change of our dC₁₀Cl with other reported colossal barocaloric materials.

The materials include 3D plastic crystals PG, NPG, NPA, spin-crossover Fe₃(bntrz)₆(tcnset)₆, substituted adamantane Br-adamantane, Cl-adamantane and hybrid organic–inorganic perovskite (C_nH_2n+1NH₃)₂MnCl₄ (n = 9, 10)^,,,–. Light color denotes the entropy change ΔS of phase transition, while dark color marks the reversible entropy change ΔS_r driven by a hydrostatic pressure of 0.1 GPa.

Fig. 4. The direct measurement of adiabatic temperature change ΔT_ad driven by pressure in dC₁₀Cl.

a ΔT_ad induced by pressure of 0.1 GPa. b Schematic illustration of temperature change and pressure evolution during the adiabatic test, containing the processes of pressurization (press-) and depressurization (depress-). c Cycle measurements of ΔT_ad induced by pressure of 0.1 GPa. d Schematic diagram of direct measurement of ΔT_ad, where the sample’s image is for indication only, and the real image of single crystal sample is shown at top right.

Fig. 5. Powder x-ray diffraction result of the (CH₃–(CH₂)₉)₂NH₂Cl.

a Refinement result of diffraction pattern at 300 K for low-temperature-state in (CH₃–(CH₂)₉)₂NH₂Cl. b Le Bail method fitting result of diffraction pattern at 330 K for high-temperature-state in (CH₃–(CH₂)₉)₂NH₂Cl, where the observed (black), calculated patterns (red), their difference (green), peak positions (purple bar), background (blue), and error factor Rwp are provided. c The evolution of lattice parameters and unit cell volume with temperature in (CH₃–(CH₂)₉)₂NH₂Cl. d Schematic illustration of layer spacing expansion across the phase transition in (CH₃–(CH₂)₉)₂NH₂Cl.

Fig. 6. MD simulation on the (CH₃–(CH₂)₉)₂NH₂Cl.

MD results of molecular crystal structure are shown in (a) for 300 K and (b) for 500 K. c The radial distribution function of carbon atoms at variable temperatures in the range of 0–20 Å. d Enlarged zone of 5–20 Å from (c). e The schematic demonstration of short-range order of carbon chain. f The schematic demonstration of distribution disorder of carbon chain at long-range level.

Fig. 7. The distribution of C–C–C–C dihedral angle composed of 3 C-C bonds from the MD simulation for (CH₃–(CH₂)₉)₂NH₂Cl.

a The dihedral angle distribution at variable temperatures. b The schematic relation between the carbon chain conformer and the dihedral angle.

Fig. 8. Temperature-variable infrared spectra of (CH₃–(CH₂)₉)₂NH₂Cl.

a The interchain and interlayer vdW interactions and the hydrogen bond interaction of organic chains with Cl^- anions in (CH₃–(CH₂)₉)₂NH₂Cl. b Temperature-variable infrared spectra in the range of 1300–1650 cm⁻¹. c Temperature-dependent vibration frequency of specific vibration band in (b). d, e Temperature-variable infrared spectra in the range of 2300–2700 cm⁻¹ and 2700–3000 cm⁻¹, respectively.