Effect of structural variation in biomass-derived nonfluorinated ionic liquids electrolytes on the performance of supercapacitors

There is a growing interest in sustainable and high performance supercapacitors(SCs) operating at elevated temperatures as they are highly demanded in heat-durable electronics. Here, we present a biomass-derived nonfluorinated ionic liquid(IL) [P_(4444)][HFuA] and its structural analogue [P_(4444)][TpA]as electrolytes for supercapacitors comprising multiwall carbon nanotubes and activated charcoal(MWCNTs/AC) mixed carbon composite electrodes. A detailed investigation of the effect of scan rate, temperature, potential window and orientation of ions on the electrodes surfaces is performed. The supercapacitors exhibited relatively lower specific capacitance for both [P_(4444)][HFuA] and [P_(4444)][TpA] ILs at room temperature. However, the specific capacitance has significantly increased with an increase in temperature and potential window. The equivalent serie resistances of the SCs is deceased with increasing temperatures, which is a result of improved ionic conductivities of the IL electrolytes. In CV cycling at60 °C, the capacitor with [P_(4444)][HFuA] IL-based electrolyte retained about 90% of its initial capacitance,while the capacitor with [P_(4444)][TpA] IL-based electrolyte retained about 83% of its initial capacitance.Atomistic computations revealed that the aromatic [FuA]^(-) and [TpA]^(-) anions displayed perpendicular distribution that can effectively neutralize charges on the carbon surfaces. However, the [HFuA]-anion exhibited somewhat tilted configurations on the carbon electrode surfaces, contributing to their outstanding capacitive performance in electrochemical devices.

Ionic liquids on uncharged and charged surfaces: In situ microstructures and nanofriction

In situ changes in the nanofriction and microstructures of ionic liquids(ILs)on uncharged and charged surfaces have been investigated using colloid probe atomic force microscopy(AFM)and molecular dynamic(MD)simulations.Two representative ILs,[BMIM][BF_(4)](BB)and[BMIM][PF_(6)](BP),containing a common cation,were selected for this study.The torsional resonance frequency was captured simultaneously when the nanoscale friction force was measured at a specified normal load;and it was regarded as a measure of the contact stiffness,reflecting in situ changes in the IL microstructures.A higher nanoscale friction force was observed on uncharged mica and highly oriented pyrolytic graphite(HOPG)surfaces when the normal load increased;additionally,a higher torsional resonance frequency was detected,revealing a higher contact stiffness and a more ordered IL layer.The nanofriction of ILs increased at charged HOPG surfaces as the bias voltage varied from 0 to 8 V or from 0 to−8 V.The simultaneously recorded torsional resonance frequency in the ILs increased with the positive or negative bias voltage,implying a stiffer IL layer and possibly more ordered ILs under these conditions.MD simulation reveals that the[BMIM]+imidazolium ring lies parallel to the uncharged surfaces preferentially,resulting in a compact and ordered IL layer.This parallel“sleeping”structure is more pronounced with the surface charging of either sign,indicating more ordered ILs,thereby substantiating the AFM-detected stiffer IL layering on the charged surfaces.Our in situ observations of the changes in nanofriction and microstructures near the uncharged and charged surfaces may facilitate the development of IL-based applications,such as lubrication and electrochemical energy storage devices,including supercapacitors and batteries.

Probing the nanofriction of non-halogenated phosphonium-based ionic liquid additives in glycol ether oil on titanium surface

The nanofrictional behavior of non-halogentated phosphonium-based ionic liquids(ILs)mixed with diethylene glycol dibutyl ether in the molar ratios of 1:10 and 1:70 was investigated on the titanium(Ti)substrate using atomic force microscopy(AFM).A significant reduction is observed in the friction coefficientμfor the IL-oil mixtures with a higher IL concentration(1:10,μ~0.05),compared to that for the lower concentration 1:70(μ~0.1).AFM approaching force–distance curves and number density profiles for IL-oil mixtures with a higher concentration revealed that the IL preferred to accumulate at the surface forming IL-rich layered structures.The ordered IL-rich layers formed on the titanium surface facilitated the reduction of the nanoscale friction by preventing direct surface-to-surface contact.However,the ordered IL layers disappeared in the case of lower concentration,resulting in an incomplete boundary layers,because the ions were displaced by molecules of the oil during sliding and revealed to be less efficient in friction reduction.