FY2010 - Oak Ridge National Laboratory

Director’s R&D Fund—
Ultrascale Computing and Data Science
Nature Materials 7, 845 (2008) and Chemical Society Reviews 38, 2520 (2009)]. A particularly
appealing feature of our approach is that it can be easily and effectively applied to any pore size and any
type of electrolyte. Its predictive power has now been used to devise a new type of supercapacitor (an
exohedral capacitor), as is detailed by Huang et al. (2010a). Impressively, this novel predicted behavior
for exohedral capacitance was subsequently validated by experimental studies [see Nature Nanotech 5,
651 (2010)]. Our related work was also featured on the cover of the Journal of Materials Research (Huang
et al. 2010b).
Second, we proposed modeling the explicit dynamics of the solvation/desolvation processes. This work is
still under way, but considerable progress was made. We have identified realistic atomistic models for
porous materials to be used as templates for studying dynamics and have carried out large size and
timescale molecular dynamics (MD) runs for a number of these. Additionally, we have also established
collaboration with Professor Rui Qiao from Clemson University through the HERE program in order to
supplement our portfolio in this area. Through this collaboration, we have studied the distribution of
K ions in electrified slit-shaped micropores with pore widths (W) ranging from 9.36 to 14.7 Å using MD
simulations (Feng et al. 2010a). We have examined in detail the main factors that govern the distribution
of K ions in electrified micropores: (1) the long-range electrostatic ion-ion repulsion, which always drives
ions toward the two slit walls, (2) the nonelectrostatic ion-slit wall attractions, (3) the hydration of ions,
which drives ions toward positions where they maximize interactions with their hydration water
molecules, (4) the interactions between an ion’s hydration water molecules and their surrounding water
molecules, and (5) entropic effects that drive the ion and water inside the slit toward an uniform
distribution. The highlight of our results is that K ions form a well-hydrated single layer in the center of
negatively charged slit pores with pore width between 10 and 14.7 Å. Such an ion distribution differs
qualitatively from the prediction by the classical EDL theories and is caused primarily by the ion
hydration effects. In slits with a width of 9.36 Å, the K ions form separate layers near each slit wall. We
found that the electrostatic ion-ion repulsion plays only a minor role in such a transition. Instead, the
enthalpic effects associated with the interactions between the hydration water molecules of the K ion with
their surrounding water molecules were found to lead to this interesting behavior. Based on the K ion
distribution observed in electrified slits with 10 Å < W < 14.7 Å, we proposed a sandwich model to
predict the scaling of the slit pore capacitance as a function of its pore width. This model is shown to be
capable of predicting the anomalous enhancement of capacitance that has been experimentally observed
in micropores with similar widths. However, curvature effects are indispensable for a quantitative
description of the experimental capacitance values, further implying that the microporous carbons have a
local pore geometry that are closer to a cylinder shape rather than that of a slit.
Additionally, we have examined organic electrolytes (Feng et al. 2010b) and ionic liquids (Feng
et al. 2010c). Details of the energy barrier for entering sub-nanometer pores to in aqueous
electrolytes are given by Feng et al. (2010d) Briefly, there is an energy penalty to enter subnanometer
pores because ions have to shed part of their solvation shell. The magnitude of such an energy
penalty plays a key role in determining the accessibility and charging/ discharging of these subnanometer pores.
Atomistic simulation of Na + and Cl - ions entering a polarizable slit pore with a center-to-center width of
0.82 nm showed that the free energy penalty for these ions to enter the pore is less than 14 kJ/mol for both
Na + and Cl - ions. The surprisingly small energy penalty is caused by the van der Waals attractions between
ions and pore walls, the image charge effects, the moderate (19–26%) dehydration of the ions inside the
pore, and the strengthened interactions between ions and their hydration water molecules in the
subnanometer pore. The results provide strong impetus for further developing nanoporous electrodes
featuring subnanometer pores.
Third, we proposed to explore the role of pore chemistry beyond carbon nanopores, including effects of
chemical functionalization and other types of elements for the porous materials, including oxides.
This topic is relevant to the effect of pseudo-capacitance, where fast and reversible redox reactions
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