Halogen (iodide, I À ) 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 ZnCl 2 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 À /I 3 À , 2I À / I 2 , 2I 3 À /3I 2 and I 2 /IO 3 À pairs produce redox peaks in CV and a large Faradaic plateau in charge-discharge curves. Similarly, I À 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 À1 and 9.5 W h kg À1 in aqueous solution of 1 M H 2 SO 4 . Interestingly, nearly two-fold improved specific capacitance and energy density of 912 F g À1 and 19.04 W h kg À1 were achieved when 0.08 M KI was added in 1 M H 2 SO 4 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 H 2 SO 4(572 F g À1 , 11.6 W h kg À1 ) and 0.08 M KI added to 1 M Na 2 SO 4 (604 F g À1 , 12.3 W h kg À1 ) as electrolytes.

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Experimental results on various photophysical properties of coumarin-30 (C30) dye, namely, Stokes' shift (Deltanu), fluorescence quantum yield (Phi(f)), fluorescence lifetime (tau(f)), radiative rate constant (k(f)) and nonradiative rate constant (k(nr)), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Deltaf = [(epsilon - 1)/(2epsilon + 1) - (n(2) - 1)/(2n(2) + 1)], they show unusual deviations in nonpolar solvents at one end and in high-polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: (1) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7-NEt(2) substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone pi cloud; (2) in medium to higher polarity solvents, the dye exists in a polar intramolecular charge transfer structure, where the 7-NEt(2) group and the 1,2-benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone pi cloud; (3) in protic solvents, the dye-solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and (4) in high-polarity protic solvents, the excited C30 undergoes a new activation-controlled nonradiative deexcitation process because of the involvement of a twisted intramolecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity-dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.

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.

A hydroquinone mediated PVA-H 2 SO 4 gel electrolyte (PHHQ) and activated carbon from bio-waste were prepared for supercapacitor fabrication. PHHQ delivered a higher capacitance (941 F g 21 at 1 mA cm 22 ) and energy density (20 Wh kg 21 at 0.33 W g 21 ) than the PVA-H 2 SO 4 gel electrolyte (425 F g 21 at 1 mA cm 22 , 9 Wh kg 21 at 0.33 W g 21 ).