Abstract
We probed electrochemical ion storage in single-walled carbon nanotubes (SWCNTs) of different diameters in two different organic electrolytes using electrochemical quartz crystal microbalance (EQCM) tracking. The measurements showed that charge storage probed by cyclic voltammetry did not deteriorate when steric effects seemed to hinder the accessibility of counter-ions into SWCNTs, and instead proceeded predominantly by co-ion desorption, as was shown by the decrease in the electrode mass probed by EQCM. The dominant mechanism correlated with the SWCNT diameter/ion size ratio; counter-ion adsorption dominated in the whole potential range when the diameter of SWCNTs was comparable to the size of the largest ion, whereas for larger diameters the charge increase coincided with a decrease in the electrode mass, indicating the dominance of co-ion desorption. The dominance of co-ion desorption was not observed in activated carbon, nor was it previously reported for other carbon materials, and is likely switched on because the carrier density of SWCNT increases with applied potential, and maintains the electrode capacity by co-ion desorption to overcome the steric hindrances to counter-ion adsorption.
Highlights
The decrease in frequency on both sides of the center of the grip correlates with the increase in the anodic and cathodic potential on the cyclic voltammogram, and indicates that the charge storage mechanism is dominated by counter-ion adsorption in the pores of the electrode, the contribution of co-ion desorption is possible as implied by the slight misalignment of the potential of zero charge PZC and potential of zero mass change PZM
The capacitance on the voltammogram continues to increase, indicating charge storage via an alternative or additional mechanism, that is, anion desorption or migration away from the negatively charged electrode. Such co-ion predominance has not been reported previously for other porous carbons like activated carbon and carbide-derived carbon,[11,12,16] so we hypothesize that the dominance of charge storage by co-ion desorption is unlikely to have been driven by the increase in applied potential, but rather by the additional force arising from the increase in charge carrier density of single-walled carbon nanotubes (SWCNTs) at potentials beyond the dumbbell grip
The unique and potential-dependent carrier density in SWCNTs plays a role in maintaining the electrode capacitance even when steric hindrance may prevent counter-ion adsorption inside the 30256 | RSC Adv., 2021, 11, 30253–30258
Summary
Electric double-layer capacitors (EDLCs) or supercapacitors have been the focus of decades of research aiming to boost their energy density to levels capable of accommodating renewable energy sources like solar and wind energy.The quest towards high-energy density supercapacitors has relied on the development of superior electrode materials, and the deepening of understanding of the charge storage process on the electrode/electrolyte interface, either through theoretical modelling or experimental coupling of electrochemical charging with diverse in situ tracking techniques.[1,2,3,4,5,6,7,8,9,10] One useful device used for in situ tracking is the electrochemical quartz crystal microbalance (EQCM), in which the generation of alternating current across a piezoelectric quartz crystal causes its vibration with a resonance frequency whose magnitude is sensitive to mass adhering onto the crystal. The decrease in frequency (increase in electrode mass) on both sides of the center of the grip correlates with the increase in the anodic and cathodic potential on the cyclic voltammogram, and indicates that the charge storage mechanism is dominated by counter-ion adsorption in the pores of the electrode, the contribution of co-ion desorption is possible as implied by the slight misalignment of the potential of zero charge PZC and potential of zero mass change PZM.
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