We have examined aqueous electrolyte solutions within a family of carbon pore geometries using classical molecular dynamics simulations.  The electrode pores are modeled as planar graphite sheets and carbon nanotubes with interstices ranging from one-half to four nanometers.  Under moderate confinement, the fluid structure facilitates focused ion motion along the pore, while motion is hindered normal to the interface.  We find that severely confining the system leads to decreased ion diffusivities in all directions.  The ions are fully hydrated in all but the smallest pores, with those sizes showing increased ion pairing.  Electrified interfaces are found to induce slight changes to the fluid structure.  Charge separation perpendicular to the surface is enhanced for both water and the brine solutions compared to the uncharged pore results, resulting from water molecule rotation in response to the surface charge.  The specific charge structuring plays a significant role in determining the width of the interfacial region, and we can relate capacitance to this molecular level structuring.   Our simulations indicate that water behavior, particularly ion hydration properties and water-water hydrogen bonding, drives the overall system behavior.  This work has elucidated the molecular origins of energy and power of aqueous electrolyte supercapacitors.  More generally, our results are consistent with the premise that supercapacitor properties are driven by interfacial phenomena.

Interfacial structure and confinement effects of aqueous electrolytes in carbon pores

247th National Meeting (2014)

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