D. Kalpana

Microwave synthesis of graphene/magnetite composite electrode material for symmetric supercapacitor with superior rate performance

Pristine Fe3O4 and Fe3O4–graphene composites were synthesized by using a green and low cost urea-assisted microwave irradiation method and were utilized as electrode materials for symmetric supercapacitor applications. The Fe3O4–graphene symmetric cell exhibited a better electrochemical performance than that of the Fe3O4 cell with enhanced rate performances. The Fe3O4–graphene symmetric cell delivered a stable discharge capacitance, energy and power densities of about 72 F g−1, 9 Wh kg−1 and 3000 W kg−1, respectively at 3.75 A g−1 current density over 100 000 cycles between 0–1 V. The impedance studies also suggested that the Fe3O4–graphene symmetric cell showed lower resistance and high conductivity due to the small particle size, large surface area and good interaction between Fe3O4 particles and graphene layers.

Effect of pH on the sonochemical synthesis of BiPO4 nanostructures and its electrochemical properties for pseudocapacitors.

Using sonochemical method, BiPO4 nanocrystals were prepared at different pH conditions (pH-1, 3, 5, 7, 9 & 12) for the possible applications of pseudocapacitor electrodes. The prepared BiPO4 nanocrystals belong to monoclinic structure with P21 space group. The SEM image revealed that the particles changed from irregular coarse shape into rod like structure (pH-1 to 7) which finally collapsed into irregular aggregates (pH-9 to pH-12). The observed spot patterns from SAED inferred the polycrystalline nature of the material. The electrochemical performance of the synthesized BiPO4 in various ultrasound irradiation conditions such as irradiation time (30min, 1h, 2h and 3h) and ultrasonication power (40%, 50%, 60% and 70% of instrumental power) was studied. A maximum specific capacitance of 1052F/g (pH-7 at 2mV/s) was observed for the BiPO4 prepared in the ultrasonication reaction condition of 2h with 60% power. Also the obtained specific capacitance was high compared with the conventional precipitation method (623F/g at 2mV/s) that revealed the prominence of sonication method. Similarly, BiPO4 prepared at pH-7 delivered a maximum specific capacitance of 302F/g at 2mA/cm(2) calculated from galvanostatic charge-discharge (GCD) method than the other pH conditions. However, the cycling stability of BiPO4 (pH-7) was not appreciable even for 200 cycles. So, attempts were taken to enhance the cycling stability of the material by employing various carbon matrices such as acetylene black, activated carbon and MWCNT instead of carbon black during electrode preparation. BiPO4 material with activated carbon delivered good capacitance retention compared with other carbon matrices. This enhanced electrochemical performance of BiPO4 (pH-7) using activated carbon matrix inferred that it could be utilized as efficient negative electrode material for pseudocapacitors.

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.

Reply to the ‘Comment on “Microwave synthesis of graphene/magnetite composite electrode material for symmetric supercapacitor with superior rate performance”’ by Rajaperumal M., RSC Adv., 2017, 7, DOI: 10.1039/c7ra04129b

First of all, this work was completed at the Chonnam National University, Korea by the rst author of this paper Mr Karthikeyan. (1) The main scope of our work is to facilitate the synthesis of Fe3O4–graphene (Fe–GNS) nanoparticles using a simple and green synthesis method and to study the effect of the preparation technique on their capacitive performance. Since the preparation conditions affect the electrochemical performance of any cathode materials (Fe3O4-conductive carbon composites have already been prepared using various methods), a novel microwave method was adopted to fabricate the Fe–GNS composites and their application as capacitors studied. In our work, we have achieved excellent capacitive performance of the Fe–GNS system, which was attributed to its morphological features (explained well in the manuscript itself). (2) Regarding the weight, the weight of the active material in the electrode was 0.3125 mg. The weight difference was about 0.4 mg and the active material in the electrode was about 0.3125 ( 79 wt%). (3) With reference to the article published in J. Mater. Chem., 2011, 21, 3422–3427, they have achieved 480 F g 1 at 5 A g 1 as shown in Fig. 3b. As you can see from Fig. 3b, the discharge time could be 43 s, voltage is 1 V and current is 5 A g 1 and the authors reported the capacitance of 480 F g . (4) About your question on capacitive performance, the rate performance of the Fe–GNS was resulted from its morphological feature as explained well in the paper. Since the open system formed during the synthesis of Fe–GNS can accommodate more electrolyte within the composite structure, providing a exible structure against inherent mechanical stresses formed during the charge discharge at