Raising the operating temperature of layered perovskite phase-change materials beyond 100 °C via alkylammonium extension

Abstract

Two-dimensional halide perovskites hold promise for thermal energy storage and barocaloric cooling and heating. The rich chemical flexibility of halide perovskites containing alkylammonium cations has not yet been fully scanned, especially for medium-temperature operations (80–150 °C). This temperature range is generating interest in the industry sector, where waste heat recovery and sustainable heat pumping are in high demand. Here, we expand the operational temperature of this material class by synthesizing two-dimensional halide perovskites with an increased number of carbons (n) in the alkylammonium cations (C_nH_2n+1NH₃) up to n = 22. We find that the previously unreported perovskites, (C_nH_2n+1NH₃)₂CuX₄ (X = Cl, Br; n = 18–22, even), display phase transitions in the 90–110 °C range, exceeding those of the related compounds by 20–30 °C. Furthermore, bromide and chloride compounds display unexpected differences in their structural and thermodynamic features. Depending on the halide, single-crystal X-ray structures reveal different conformations in the carbon atoms closest to the ammonia group and tilting angles of the organic cations with respect to the inorganic sublattice, resulting in denser packing for the bromide derivatives. Also, the bromide perovskites display a main phase transition with significant latent heat, whereas the chloride perovskites exhibit a sequence of less energetic transitions. The phase transitions in the 90–110 °C range, low hysteresis (0.1–1 °C), and high-pressure sensitivity of the transition (dT/dp ∼ 300 °C GPa⁻¹) render them interesting candidates for both barocaloric and thermal energy storage applications at medium temperatures.