Electrosorption of monovalent alkaline metal ions onto highly ordered mesoporous titanium dioxide nanotube electrodes

Molecular modeling of electrical double layer (EDL) structure and electrosorption of ions in pores predicts the on-set of competitive entropic and energetic effects, such as electrosorption capacity mainly determined by size exclusion effects rather than applied electrical potential. Classical EDL Theory does not predict these effects. Although size-exclusion effects have been used to explain discrepancies between experimental observations and Classical EDL Theory, direct experimental validation of molecular modeling predictions have yet to be achieved. The main objective of the present work was to devise and test a model system that can be used to experimentally measure size-exclusion effects for future validation of molecular simulations. This work is a first attempt to bridge the gap between molecular modeling predictions of EDL structure and experimentally-measured macroscopic properties of the EDL.Highly-uniform, titanium dioxide nanotubes with three pore sizes (36.8, 41.4 and 44.9 nm) were used as model electrodes to study the interactive effects of pore diameter, applied potential and ion size on the electrosorption of three monovalent alkaline-metal cations (Li+, Na+, Cs+).The proportionality of EDL electrical capacitance and ion-electrosorption capacity to surface area and applied potential predicted by Classical EDL Theory did not hold for all combinations of pore sizes and applied potentials examined in this work, and electrosorption capacity clearly depended on pore diameter rather than specific surface area, and on hydrated-ion radius. The measured electrosorption capacity trend in most cases corresponded to Cs+ > Na+ > Li+. This trend follows the hydrated radius size. Size exclusion effects on EDL capacitance and electrosorption capacity triggered by hydrated ion-size and pore diameter observed in this work were in qualitative agreement with molecular modeling predictions.

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