Tailoring high-field energy storage and dual-mode electrocaloric response in relaxor ferroelectric thin film

Ferroelectric and antiferroelectric thin and thick films with strong electrocaloric (EC) responses are attractive for solid-state cooling. In this study, 0.75PbMg_1/3Nb_2/3O₃-0.25PbTiO₃ (PMN-25PT) relaxor ferroelectric thin films deposited on LSAT substrates by pulsed laser deposition exhibit a giant negative EC effect near 150 °C, with a maximum temperature change of -38.3 K and an entropy change of -29.4 J kg^-1 K^-1. A large positive EC effect is also observed near 120 °C, yielding ΔT = 33.4 K and ΔS = 27.5 J kg^-1 K^-1, comparable to the best reported values. The films show nanoscale columnar grains that promote polar nanoregions, leading to high polarization, large dielectric breakdown strength, and a diffuse, frequency-dependent dielectric response. Polarization-electric field loops measured from 30 °C to 190 °C reveal the excellent thermal stability of energy storage, which remains robust after 10⁸ charge-discharge cycles. Moreover, the films demonstrate efficient harvesting of low-grade waste heat, achieving an energy conversion density of ∼18.7 J cm^-3 per cycle over 0-2.5 MV cm^-1 and a wide temperature range of 30-140 °C, as evaluated using pyroelectric Olsen cycles. These results highlight PMN-25PT thin films as promising candidates for advanced energy storage and electrocaloric cooling applications.