The electrocaloric effect, or ECE, is deceptively simple in principle. When an electric field is applied to a dielectric material, its internal polarization increases, which reduces the entropy of the dipoles that make up that polarization. Because the total entropy of the material must be conserved when the field is applied adiabatically, the temperature of the material rises. Remove the field and the temperature drops again. This reversible temperature change, typically a few degrees in favorable materials, forms the basis of an entirely solid-state refrigeration cycle. In the cycle described in the review, the material first absorbs heat from a load while cold, is isolated and heated by the field, dumps that heat to a sink, and then cools again when the field is removed. This Brayton-type cycle repeats continuously, with no moving fluids and no greenhouse gases.

The phenomenon itself is far from new. The concept dates to 1878, and Kobeco and Kurtschatov first quantified it experimentally in 1930 in Rochelle salt. For decades, however, the observed temperature changes hovered around one degree Celsius, far too small for practical refrigeration. Early research focused on lead-based ferroelectric compounds, which delivered the strongest responses, including a landmark 12-degree temperature change in thin films of lead zirconate titanate reported in 2006. But lead-based materials carry serious liabilities: they are toxic, thermally unstable under prolonged heating, difficult to fabricate into devices, expensive to produce, and prone to polarization fatigue. European hazardous substance regulations have effectively mandated a search for alternatives, and that search now defines the field.

The review, authored by Sana Ullah, Hassan Raza, Awais Akhtar, Guangping Zheng, and Abhijit Pramanick, systematically catalogs the lead-free families now under investigation, from bulk ceramics and single crystals to thin films, multilayer structures, and polymers. Among bulk ceramics, barium titanate stands out as the workhorse. Reported ECE strength reaches 0.48 kelvin centimeters per kilovolt, with a maximum temperature change of 4.8 degrees under a field of 10 kilovolts per centimeter at 136 degrees Celsius. Substituting strontium into barium titanate to form barium strontium titanate allows researchers to tune the Curie temperature and broaden the operating window, though excessive strontium content actually suppresses the electrocaloric response, a reminder that compositional engineering is a delicate balancing act.

Doping strategies have proven especially powerful. Manganese-doped barium strontium titanate ceramics fabricated by spark plasma sintering achieved an entropy change of 4.77 joules per kilogram-kelvin, roughly 45 percent higher than undoped equivalents, thanks to dramatically improved electrical resistivity. Bismuth sodium titanate based compositions offer a different route: in these materials, applied fields induce transitions between ferroelectric and antiferroelectric states, and the resulting structural rearrangements produce substantial cooling. Bismuth sodium titanate barium titanate ceramics delivered a 2.1-degree temperature change under 60 kilovolts per centimeter, while barium calcium zirconium titanate ceramics, notable for their slim hysteresis loops, showed consistent electrocaloric behavior across a remarkably wide temperature range from about 28 to 105 degrees Celsius.

The review also highlights exciting developments in low-dimensional and hybrid materials that suggest the field is expanding well beyond classical ceramics. The layered van der Waals ferroelectric copper indium phosphorus sulfide, CuInP2S6, exhibits an entropy change of about 5.8 joules per kilogram-kelvin and a 3.3-kelvin temperature change near 315 kelvin under modest fields, demonstrating that two-dimensional ferroelectrics can deliver meaningful electrocaloric effects at room temperature. Scanning electrocaloric thermometry has even resolved local cooling in atomically thin indium selenide crystals. Layered organic-inorganic hybrid perovskites, typically studied for optoelectronics, have joined the club: one lead-containing hybrid chloride compound showed an 11-kelvin electrocaloric temperature change near ambient conditions, opening a new design space where molecular and inorganic sublattices can be engineered independently.

Films and multilayer architectures represent the pragmatic path toward devices, because thin geometries tolerate electric fields hundreds of times stronger than bulk ceramics can survive. Thin films of barium titanate have reached electrocaloric changes of nearly 12 degrees, rivaling lead-based analogues. Strontium bismuth tantalate films, measured for the first time for giant ECE, delivered 4.93 degrees at 600 kilovolts per centimeter. Perhaps most striking, bilayer thin films combining barium zirconium titanate and bismuth sodium titanate barium titanate produced a temperature change of roughly 23 kelvin near room temperature under a moderate field, attributed to interface-charge engineering between the two layers. Multilayer stacks with internal electrodes push further still: barium titanate multilayers reached 4 degrees at 352 kilovolts per centimeter in direct measurements, exceeding thermodynamic predictions, while eco-friendly bilayer thick films of barium zirconium titanate stacked on barium tin titanate achieved about 5.2 kelvin with an entropy change of 6.9 joules per kilogram-kelvin near zero degrees Celsius.

Polymers occupy the opposite end of the flexibility spectrum and may prove equally transformative. Polyvinylidene fluoride based copolymers and terpolymers, including P(VDF-TrFE) and its chlorofluoroethylene-modified terpolymer, exhibit giant electrocaloric responses, with copolymer films reaching 35 degrees at 1800 kilovolts per centimeter near a first-order ferroelectric transition, and Langmuir-Blodgett grown films recording up to 21 degrees. Nanocomposites push performance further: adding boron nitride nanosheets and barium strontium titanate nanostructures to a terpolymer matrix produced a colossal 50.5-degree temperature change at 2500 kilovolts per centimeter, while two-dimensional graphitic carbon nitride fillers enabled strong negative electrocaloric effects useful for heating-to-cooling switching. The review cautions, however, that indirect ECE estimates via Maxwell relations can seriously mislead in relaxor polymers, where polar nanoregions and non-ergodic dynamics violate the equilibrium assumptions underlying the equations, sometimes understating the true response by large margins.

Complementing the experimental landscape, the review surveys the theoretical machinery now guiding design, including Landau-Ginzburg-Devonshire phenomenology, phase-field simulation, and first-principles molecular dynamics. These tools have explained how clamping, misfit strain, domain structure, and surface effects modulate electrocaloric response, predicted giant effects in ferroelectric nanotubes and tunnel junctions, and revealed the coexistence of positive and negative electrocaloric regimes within single materials. The remaining hurdles are sobering: most high-performance materials still require fields far above what consumer electronics can supply, cooling peaks occupy narrow temperature windows, and long-term reliability under cyclic operation remains unproven at scale. Yet the trajectory is unmistakable. With compositional engineering, defect control, and multilayer architectures steadily lowering the field thresholds, and multifunctional designs that couple cooling with energy storage now emerging, lead-free electrocaloric materials are moving from laboratory curiosities toward the practical, compact, and genuinely green cooling systems that a warming world urgently needs.

Subject of Research: The electrocaloric effect in lead-free ferroelectric materials for solid-state cooling applications

Article Title: A comprehensive review of the electrocaloric effect in lead free ferroelectrics for solid state cooling

Article References: Ullah, S., Raza, H., Akhtar, A., Zheng, G., & Pramanick, A. (2026). A comprehensive review of the electrocaloric effect in lead free ferroelectrics for solid state cooling. Discover Industrial Chemistry and Materials, 1(1), Article 2. https://doi.org/10.1007/s44508-026-00001-2

Image Credits: AI Generated

DOI: 10.1007/s44508-026-00001-2

Keywords: electrocaloric effect, solid-state cooling, lead-free ferroelectrics, barium titanate, relaxor ferroelectrics, ferroelectric polymers, PVDF copolymers, multilayer ceramics, phase transitions, Maxwell relations, refrigerant-free refrigeration, energy efficiency