S. T. Senthilkumar

Electric double layer capacitor and its improved specific capacitance using redox additive electrolyte

Halogen (iodide, I−) added aqueous electrolyte facilitates the capacitive behaviour of biomass derived activated carbon based electric double layer capacitors. To produce economically viable electrodes in large scale for supercapacitors (SCs), the activated carbons (ACs) prepared from Eichhornia crassipes (common water hyacinth) by ZnCl2 activation. The prepared ACs were characterized by XRD, Raman, FT-IR and surface area, pore size and pore volume analysis. The electrochemical properties of the SCs were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling stability. The 3I−/I3−, 2I−/ I2, 2I3−/3I2 and I2/IO3− pairs produce redox peaks in CV and a large Faradaic plateau in charge–discharge curves. Similarly, I− ions improves the good ionic conductivity (lower charge transfer resistance) at the electrode/electrolyte interface which was identified through EIS studies. The calculated specific capacitance and energy density was 472 F g−1 and 9.5 W h kg−1 in aqueous solution of 1 M H2SO4. Interestingly, nearly two-fold improved specific capacitance and energy density of 912 F g−1 and 19.04 W h kg−1 were achieved when 0.08 M KI was added in 1 M H2SO4 electrolyte with excellent cycle stability over 4000 cycles. Subsequently, this improved specific capacitance and energy density was compared with 0.08 M KBr added to 1 M H2SO4 (572 F g−1, 11.6 W h kg−1) and 0.08 M KI added to 1 M Na2SO4 (604 F g−1, 12.3 W h kg−1) as electrolytes.

Redox additive/active electrolytes: a novel approach to enhance the performance of supercapacitors

Currently, supercapacitors (SCs) are a promising field in the area of energy storage devices. In the last few decades, different types of carbon based materials with suitable surface modifications, metal oxides, metal hydroxides, conducting polymers and their various composites have been used as electrodes to improve the energy performance of SCs. In addition, different technologies like asymmetric and hybrid systems have also been introduced. Interestingly, another alternative approach has been proposed recently by a few research groups, wherein electrolytes (liquid and polymer) can enhance the performance of the SCs via redox reactions at the electrode–electrolyte interface, by introducing redox additives or mediators in the electrolytes. The main advantage of this new technique is its simple and safe preparation method, along with cost effectiveness compared to the preparation of some active electrode materials. Hence it is believed that identification of suitable redox additives or species in electrolytes will be a hotspot in the field of SCs in the coming years.

High Performance Solid-State Electric Double Layer Capacitor from Redox Mediated Gel Polymer Electrolyte and Renewable Tamarind Fruit Shell Derived Porous Carbon

The activated carbon was derived from tamarind fruit shell and utilized as electrodes in a solid state electrochemical double layer capacitor (SSEDLC). The fabricated SSEDLC with PVA (polyvinyl alcohol)/H2SO4 gel electrolyte delivered high specific capacitance and energy density of 412 F g(-1) and 9.166 W h kg(-1), respectively, at 1.56 A g(-1). Subsequently, Na2MoO4 (sodium molybdate) added PVA/H2SO4 gel electrolyte was also prepared and applied for SSEDLC, to improve the performance. Surprisingly, 57.2% of specific capacitance (648 F g(-1)) and of energy density (14.4 Wh kg(-1)) was increased while introducing Na2MoO4 as the redox mediator in PVA/H2SO4 gel electrolyte. This improved performance is owed to the redox reaction between Mo(VI)/Mo(V) and Mo(VI)/Mo(IV) redox couples in Na2MoO4/PVA/H2SO4 gel electrolyte. Similarly, the fabricated device shows the excellent capacitance retention of 93% for over 3000 cycles. The present work suggests that the Na2MoO4 added PVA/H2SO4 gel is a potential electrolyte to improve the performance instead of pristine PVA/H2SO4 gel electrolyte. Based on the overall performance, it is strongly believed that the combination of tamarind fruit shell derived activated carbon and Na2MoO4/PVA/H2SO4 gel electrolyte is more attractive in the near future for high performance SSEDLCs.

Redox additive aqueous polymer gel electrolyte for an electric double layer capacitor

A hydroquinone mediated PVA–H2SO4 gel electrolyte (PHHQ) and activated carbon from bio-waste were prepared for supercapacitor fabrication. PHHQ delivered a higher capacitance (941 F g−1 at 1 mA cm−2) and energy density (20 Wh kg−1 at 0.33 W g−1) than the PVA–H2SO4 gel electrolyte (425 F g−1 at 1 mA cm−2, 9 Wh kg−1 at 0.33 W g−1).

Improved performance of electric double layer capacitor using redox additive (VO2+/VO2+) aqueous electrolyte

Electric double layer capacitors (EDLCs) were fabricated using biomass derived porous activated carbon as electrode material with 1 M H2SO4 and VOSO4 added 1 M H2SO4 as electrolytes. Here, VOSO4 was used as redox additive to improve the overall performance of EDLC. As expected, the VOSO4 electrolyte showed ∼43% of improved specific capacitance of 630.6 F g−1 at 1 mA cm−2 compared to pristine 1 M H2SO4 (440.6 F g−1) due to the contribution of VO2+/VO2+ redox reaction at the electrode–electrolyte interface. Possible redox reaction mechanism of VO2+/VO2+ pair is also briefly illustrated. The good cycling performance of 97.57% capacitance retention was observed even after 4000 cycles. For comparison, the polymer gel electrolyte (PVA/VOSO4/H2SO4) was also prepared and then the performance of the fabricated EDLCs was studied. Overall, these findings could open up a simple and cost effective way to improve the performance of EDLCs significantly.

The biomass derived activated carbon for supercapacitor

In this work, the activated carbon was prepared from biowaste of Eichhornia crassipes by chemical activation method using KOH as the activating agent at various carbonization temperatures (600 °C, 700 °C and 800 °C). The disordered nature, morphology and surface functional groups of ACs were examined by XRD, SEM and FT-IR. The electrochemical properties of AC electrodes were studied in 1M H2SO4 in the potential range of −0.2 to 0.8 V using cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques in a three electrode system. Subsequently, the fabricated supercapacitor using AC electrode delivered the higher specific capacitance and energy density of 509 F/g at current density of 1 mA/cm2 and 17 Wh/kg at power density of 0.416 W/g.In this work, the activated carbon was prepared from biowaste of Eichhornia crassipes by chemical activation method using KOH as the activating agent at various carbonization temperatures (600 °C, 700 °C and 800 °C). The disordered nature, morphology and surface functional groups of ACs were examined by XRD, SEM and FT-IR. The electrochemical properties of AC electrodes were studied in 1M H2SO4 in the potential range of −0.2 to 0.8 V using cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques in a three electrode system. Subsequently, the fabricated supercapacitor using AC electrode delivered the higher specific capacitance and energy density of 509 F/g at current density of 1 mA/cm2 and 17 Wh/kg at power density of 0.416 W/g.