Kuo Han

High Energy and Power Density Capacitors from Solution‐Processed Ternary Ferroelectric Polymer Nanocomposites

Concurrent improvements in dielectric constant and breakdown strength are attained in a solution-processed ternary ferroelectric polymer nanocomposite incorporated with two-dimensional boron nitride nanosheets and zero-dimensional barium titanate nanoparticles that synergistically interact to enable a remarkable energy-storage capability, including large discharged energy density, high charge-discharge efficiency, and great power density.

Solution-processed ferroelectric terpolymer nanocomposites with high breakdown strength and energy density utilizing boron nitride nanosheets

The development of high-performance capacitive energy storage devices is of critical importance to address an ever-increasing electricity need. The energy density of a film capacitor is determined by the dielectric constant and breakdown strength of dielectric materials. With the highest dielectric constant among the known polymers, poly(vinylidene fluoride)-based ferroelectric terpolymers are of great potential for high energy density capacitors. However, their energy storage capability has long been limited by the relatively low breakdown strength. Here we demonstrate remarkable improvements in the energy density and charge–discharge efficiency of the ferroelectric terpolymers upon the incorporation of ultra-thin boron nitride nanosheets (BNNSs). It is found that BNNSs function as a robust scaffold to hamper the onset of electromechanical failure and simultaneously as an efficient insulating barrier against electrical conduction in the resulting polymer nanocomposites, resulting in greatly enhanced breakdown strength. Of particular note is the improved thermal conductivity of the terpolymer with the introduction of BNNSs; this is anticipated to benefit the stability and lifetime of polymer capacitors. This work establishes a facile, yet efficient approach to solution-processable dielectric materials with performance comparable or even superior to those achieved in the traditionally melt-extruded ultra-thin films.

Ferroelectric polymer networks with high energy density and improved discharged efficiency for dielectric energy storage

Ferroelectric polymers are being actively explored as dielectric materials for electrical energy storage applications. However, their high dielectric constants and outstanding energy densities are accompanied by large dielectric loss due to ferroelectric hysteresis and electrical conduction, resulting in poor charge-discharge efficiencies under high electric fields. To address this long-standing problem, here we report the ferroelectric polymer networks exhibiting significantly reduced dielectric loss, superior polarization and greatly improved breakdown strength and reliability, while maintaining their fast discharge capability at a rate of microseconds. These concurrent improvements lead to unprecedented charge-discharge efficiencies and large values of the discharged energy density and also enable the operation of the ferroelectric polymers at elevated temperatures, which clearly outperforms the melt-extruded ferroelectric polymer films that represents the state of the art in dielectric polymers. The simplicity and scalability of the described method further suggest their potential for high energy density capacitors.

Ferroelectric Polymer Nanocomposites for Room‐Temperature Electrocaloric Refrigeration

Solution-processable ferroelectric polymer nanocomposites are developed as a new form of electrocaloric materials that can be effectively operated under both modest and high electric fields at ambient temperature. By integrating the complementary properties of the constituents, the nanocomposites exhibit state-of-the-art cooling energy densities. Greatly improved thermal conductivity also yields superior cooling power densities validated by finite volume simulations.

Suppression of energy dissipation and enhancement of breakdown strength in ferroelectric polymer–graphene percolative composites

The percolative polymer composites have recently exhibited great potential in energy storage due to their high dielectric permittivities in the neighborhood of the percolation threshold. Yet high energy dissipation and poor voltage endurance of the percolative composites resulting from electrical conduction are still open issues to be addressed before full potential can be realized. Herein we report the percolative composites based on ferroelectric poly(vinylidene fluoride-co-chlorotrifluoroethylene) as the matrix and SiO2 coated reduced graphene oxide nanosheets as the filler. By capitalizing on the SiO2 surface layers which have high electrical resistivity and breakdown strength, the composites exhibit superior dielectric performance as compared to the respective composites containing bare reduced graphene oxide nanosheet fillers. In addition to greatly reduced dielectric loss, little change in dielectric loss has been observed within the medium frequency range (i.e. 300 kHz–3 MHz) in the prepared composites even with a filler concentration beyond the percolation threshold, indicating significantly suppressed energy dissipation and the feasibility of using the conductor–insulator composites beyond the percolation threshold. Moreover, these composites exhibit a remarkable breakdown strength of 80 MV m−1 at the percolation threshold, which far exceeds those of conventional percolative composites (lower than 0.1 MV m−1 in most cases) and thus enables the applications of the percolative composites at high electric fields. This work offers a new avenue to the percolative polymer composites exhibiting high permittivity, reduced loss and excellent breakdown strength for electrical energy storage applications.

High Energy Density and Breakdown Strength from β and γ Phases in Poly(vinylidene fluoride-co-bromotrifluoroethylene) Copolymers

Poly(vinylidene fluoride) PVDF-based copolymers represent the state of the art dielectric polymers for high energy density capacitors. Past work on these copolymers has been done with limited emphasis on the effects of copolymer composition and with a limited range of defect monomers, focusing primarily on the commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-CTFE), and poly(vinylidene fluoride-co-hexafluoropropylene), P(VDF-HFP), and the processing thereof. To expand on this area of research, copolymers of VDF and bromotrifluoroethylene (BTFE) were synthesized examining the composition range where uniaxial stretching was possible. It is found that P(VDF-BTFE) copolymers with small BTFE contents (< 2 mol %) stabilize the γ phase, compared to P(VDF-CTFE)s and P(VDF-HFP)s that are largely α phase in composition. Furthermore, different from P(VDF-CTFE)s and P(VDF-HFP)s, whose energy storage capabilities depend on the reversibility of the α to β phases transformation, high discharged energy densities (i.e., 20.8 J/cm(3) at 716 MV/m) are also achievable through the β and γ phases in P(VDF-BTFE)s without significantly reducing crystallinity and breakdown strength. This study demonstrates new avenues to the development of high energy density ferroelectric copolymers via manipulation of the γ phase through variation of the structure and content of comonomers.

Synthesis of poly(vinylidene fluoride-co-bromotrifluoroethylene) and effects of molecular defects on microstructure and dielectric properties

A series of copolymers composed of vinylidene fluoride (VDF) and bromotrifluoroethylene (BTFE) have been synthesized via suspension polymerization up to crystallinity inhibition. P(VDF-co-BTFE) copolymers exhibit different regioregularity in comparison to previously reported PVDF based copolymers owing to differences in size and reactivity of BTFE. The polymerization of the comonomers result in molecular defects that are shown to be both included (single BTFE defects) and excluded (runs of BTFE monomers) from the crystalline phase. The effects of increasing defect concentrations determined by 19F NMR were evaluated on the resulting microstructures by using Fourier transformed infrared spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction. Dielectric properties have been investigated in terms of complex permittivity as a function of frequency and temperature. The results indicate that the single BTFE defects are incorporated into the crystalline phase and destabilize the ferroelectric β phase, while the excluded defects reduce both lamellar and lateral crystallite sizes though also resulting in a significant drop in crystallinity. The excluded defects are found to expand the interlamellar region of the crystalline phase, which increases both temperature and frequency dependence of the dielectric β relaxation.

Modular synthesis and dielectric properties of high-performance fluorinated poly(arylene ether-1,3,4-oxadiazole)s

A versatile and facile synthetic approach to a series of fluorinated poly(arylene ether-1,3,4-oxadiazole)s has been described. The mild reaction conditions avoid the usage of moisture-sensitive agents and side-reactions associated with the conventional aromatic nucleophilic polycondensation. The polymers display excellent stability of the dielectric properties over a broad frequency and temperature range and high breakdown strength.

Flexible three-dimensional interconnected piezoelectric ceramic foam based composites for highly efficient concurrent mechanical and thermal energy harvesting

Flexible piezoelectric materials are pivotal to a variety of emerging applications ranging from wearable electronic devices, sensors to biomedical devices. Current ceramic-polymer composites with embedded low-dimensional ceramic fillers, though mechanically flexible, suffer from low piezoelectricity owing to the poor load-transfer efficiency that typically scales with the stiffness ratio of the polymer matrix to the ceramic fillers. Herein we introduce the scalable ceramic-polymer composites based on three-dimensional (3-D) interconnected piezoelectric microfoams. Comprehensive mechanics analyses reveal that the 3-D interconnected architecture presents a continuous pathway for load transfer to break the load-transfer scaling law seen in the conventional composites with low-dimensional ceramic fillers. The 3-D composite exhibits exceptional piezoelectric characteristics under multiple loading conditions (i.e., compression, stretching, and bending) and high mechanical durability under thousands of cycles. The 3-D composite also displays excellent pyroelectricity, thereby enabling concurrent thermal and mechanical energy scavenging. Our findings suggest an innovative material framework for high-performance energy harvesters and self-powered micromechanical devices.