U Hyeok Choi

Electro-magnetic properties of composites with aligned Fe-Co hollow fibers

A novel Fe-Co binary hollow fiber was synthesized by electroless plating using hydrolyzed polyester fiber and its anisotropy characteristic was investigated for electromagnetic wave absorbing materials. The hollow fibers in parallel with magnetic field show higher saturated magnetization of 202 emu/g at the applied magnetic field of 10 kOe and lower coercivity (27.658 Oe), compared with the random and vertical oriented hollow fibers. From complex permittivity measurement, the Fe-Co hollow fiber composites clearly display a single dielectric resonance, located at ∼14 GHz. The Fe-Co hollow fibers not only provide excellent EM properties in GHz frequency ranges, resulting mainly from the strong resonance, but also adjust the soft magnetic properties through fiber alignments. The cavitary structure of the Fe-Co hollow fibers, not only giving rise to a dielectric loss resonance and also adjusting its peak frequency, may be a pathway to useful EM wave absorptive devices in GHz frequency ranges.

Ion Conduction, Dielectric and Mechanical Properties of Epoxy-Based Solid Polymer Electrolytes Containing Succinonitrile

We use impedance spectroscopy to investigate ionic conduction and dielectric response and mechanical tests to study mechanical properties of cross-linked epoxy-based solid polymer electrolytes (SPEs) containing a mixture of succinonitrile (SN) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with different ratio of mechanically robust epoxy and ionic conducting SN/LiTFSI mixture content. Increasing SN/LiTFSI mixture content results in a proportional increase in ionic conductivity σDC, and its maximum conductivity approaches σDC∼10-4 S/cm at room temperature. This is consistent with accelerating the observed relaxation processes such as ion rearrangement and segmental motion, upon adding further SN/LiTFSI content. The combination of epoxy and SN/LiTFSI also leads to a large increase in static dielectric constant εs∼75, which is higher than the prediction from the Landau and Lifshitz mixing rule, compared to the host epoxy (εs∼4) and pure SN/LiTFSI mixture (εs∼53). On the other hand, the addition of SN/LiTFSI decreases Young’s modulus (E), compared to the neat epoxy, and approaches E∼10 MPa at room temperature, reflecting a trade-off relationship between E and σDC.

Multifunctional Epoxy-Based Solid Polymer Electrolytes for Solid-State Supercapacitors

Solid polymer electrolytes (SPEs) have drawn attention for promising multifunctional electrolytes requiring very good mechanical properties and ionic conductivity. To develop a safe SPE for energy storage applications, mechanically robust cross-linked epoxy matrix is combined with fast ion-diffusing ionic liquid/lithium salt electrolyte (ILE) via a simple one-pot curing process. The epoxy-rich SPEs show higher Youngs modulus ( E), with higher glass transition temperature ( Tg) but lower ionic conductivity (σdc) with a higher activation energy, compared to the ILE-rich SPEs. The incorporation of inorganic robust Al2O3 nanowire simultaneously provides excellent mechanical robustness ( E ≈ 1 GPa at 25 °C) and good conductivity (σdc ≈ 2.9 × 10-4 S/cm at 25 °C) to the SPE. This suggests that the SPE has a bicontinuous microphase separation into ILE-rich and epoxy-rich microdomain, where ILE continuous conducting phases are intertwined with a sturdy cross-linked amorphous epoxy framework, supported by the observation of the two Tgs and low tortuosity as well as the microstructural investigation. After assembling the SPE with activated carbon electrodes, we successfully demonstrate the supercapacitor performance, exhibiting high energy and power density (75 W h/kg at 382 W/kg and 9.3 kW/kg at 44 W h/kg). This facile strategy holds tremendous potential to advance multifunctional energy storage technology for next-generation electric vehicles.