Kaliyappan Karthikeyan

Construction of High‐Energy‐Density Supercapacitors from Pine‐Cone‐Derived High‐Surface‐Area Carbons

Very high surface area activated carbons (AC) are synthesized from pine cone petals by a chemical activation process and subsequently evaluated as an electrode material for supercapacitor applications in a nonaqueous medium. The maximum specific surface area of ∼3950 m(2)  g(-1) is noted for the material treated with a 1:5 ratio of KOH to pine cone petals (PCC5), which is much higher than that reported for carbonaceous materials derived from various other biomass precursors. A symmetric supercapacitor is fabricated with PCC5 electrodes, and the results showed enhanced supercapacitive behavior with the highest energy density of ∼61 Wh kg(-1). Furthermore, outstanding cycling ability is evidenced for such a configuration, and ∼90 % of the initial specific capacitance after 20,000 cycles under harsh conditions was observed. This result revealed that the pine-cone-derived high-surface-area AC can be used effectively as a promising electrode material to construct high-energy-density supercapacitors.

Single-step microwave mediated synthesis of the CoS2 anode material for high rate hybrid supercapacitors

A short time microwave irradiation based synthesis method of phase pure cubic CoS2 nanoparticles is reported in this study for the first time. The energy density (ED) of hybrid supercapacitors based on CoS2 as an anode having activated carbon as a cathode has been enhanced by using the higher operating potential of organic electrolytes and by increasing the concentration of the mobile ionic species at the negative electrode, in addition to the lithium ions present in the electrolyte. The specific capacitance delivered by non-lithiated CoS2 nanoflakes was 52 F g−1 at a current rate of 0.7 A g−1 between 0 and 3 V using a LiPF6-based electrolyte. Increasing the concentration of the mobile ionic species, i.e., lithium, at the anode enhanced the performance of the hybrid supercapacitor to 119 F g−1 at a current rate of 0.7 A g−1. The hierarchical arrangement of pores in the electroactive material allowed high electrolyte access and reduced the length of the ionic pathway. Consequently, the lithiated form exhibited an ED of 37 W h kg−1 with a power density of 1 kW kg−1 at a current rate of 0.7 A g−1, compared to only 15 W h kg−1 for the non-lithiated sample. Furthermore, both samples maintained superior stability over extended cycling for 10000 cycles at a very high PD of 4 kW kg−1 with a capacitance retention of 100% for the lithiated sample and 80% for the non-lithiated sample. These results will be useful in the fabrication of high ED, high rate hybrid supercapacitors for electric vehicle applications.

Fluorine‐Doped Fe2O3 as High Energy Density Electroactive Material for Hybrid Supercapacitor Applications

Nanostructured α-Fe2 O3 with and without fluorine substitution were successfully obtained by a green route, that is, microwave irradiation. The hematite phase materials were evaluated as a high-performance electrode material in a hybrid supercapacitor configuration along with activated carbon (AC). The presence of fluorine was confirmed through X-ray photoelectron spectroscopy and transmission electron microscopy. Fluorine-doped Fe2 O3 (F-Fe2 O3 ) exhibits an enhanced pseudocapacitive performance compared to that of the bare hematite phase. The F-Fe2 O3 /AC cell delivered a specific capacitance of 71 F g(-1) at a current density of 2.25 A g(-1) and retained approximately 90 % of its initial capacitance after 15 000 cycles. Furthermore, the F-Fe2 O3 /AC cell showed a very high energy density of about 28 W h kg(-1) compared to bare hematite phase (∼9 W h kg(-1) ). These data clearly reveal that the electrochemical performance of Fe2 O3 can be improved by fluorine doping, thereby dramatically improving the energy density of the system.

Surfactant-free hydrothermal synthesis of hierarchically structured spherical CuBi2O4 as negative electrodes for Li-ion hybrid capacitors

Hierarchically structured spherical CuBi2O4 particles were prepared using a facile hydrothermal method without using a surfactant over various hydrothermal reaction periods. The prepared CuBi2O4 samples were examined via X-ray diffraction (XRD), which confirmed the formation of a tetragonal crystal structure. The morphological features were analyzed using field emission scanning electron microscopy (FESEM), which elucidated the construction of the hierarchical microspherical CuBi2O4 particles. The plausible growth mechanism of the hierarchical structure was explained in terms of a time-dependent synthesis process and its crystal structure. The uniform hierarchical CuBi2O4 microspheres were used to fabricate a Li-ion hybrid capacitor (Li-HC) along with activated carbon (AC), the generated device delivers a stable specific capacitance of 26.5 F g(-1) over 1500 cycles at a high current density of 1000 mA g(-1) and a capacity retention of ∼86%. The AC/CB2 Li-ion hybrid cell exhibits high energy density and power density values of 24 W h kg(-1) and 300 W kg(-1), respectively.

High‐Power Lithium‐Ion Capacitor using LiMnBO3‐Nanobead Anode and Polyaniline‐Nanofiber Cathode with Excellent Cycle Life

LiMnBO3 nanobeads (LMB-NB) with uniform size and distribution were synthesized using a urea-assisted microwave/solvothermal method. The potential application of LMB-NBs as an anode for a lithium-ion hybrid capacitor (Li-AHC) was tested with a polyaniline-nanofiber (PANI-NF) cathode in a nonaqueous LiPF6 (1 M)-ethylene carbonate/dimethyl carbonate electrolyte. Cyclic voltammetry (CV) and charge-discharge (C/DC) studies revealed that the PANI-NF/LMB-NB cell showed an exceptional capacitance behavior between 0-3 V along with a prolonged cycle life. A discharge capacitance of about 125 F g(-1) , and energy and power densities of about 42 Wh kg(-1) and 1500 W kg(-1) , respectively, could be obtained at a current density of 1 A g(-1) ; those Li-AHC values are higher relative to cells containing various lithium intercalation materials in nonaqueous electrolytes. In addition, the PANI-NF/LMB-NB cell also had an outstanding rate performance with a capacitance of 54 F g(-1) and a power density of 3250 W kg(-1) at a current density of 2.25 A g(-1) and maintained 94% of its initial value after 30000 cycles. This improved capacitive performance with an excellent electrochemical stability could be the result of the morphological features and inherent conductive nature of the electroactive species.

Microwave synthesis of high rate nanostructured LiMnBO3 with excellent cyclic behavior for lithium ion batteries

LiMnBO3 nanobeads (LMB-NB) prepared using a urea assisted microwave irradiation method delivered 67 mA h g−1 capacity at 0.6 C rate between 1.25 and 4.8 V, showing 88% cyclability after 100 cycles. In addition, the LMB-NB electrode also showed superior rate capability with 63 and 55 mA h g−1 capacity at 1.5 and 4 C rates, which is the best ever reported rate performance for a LiMnBO3 material.