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Molecular Insights into Aqueous NaCl Electrolytes Confined within Vertically-oriented Graphenes.

Bo Z, Yang H, Zhang S, Yang J, Yan J, Cen K - Sci Rep (2015)

Bottom Line: This study, for the first time, reports the molecular dynamics (MD) simulations on aqueous NaCl electrolytes confined within VG channels with different surface charge densities and channel widths.The results are further quantified and elucidated by calculating the electrolyte density profiles.The molecular insights obtained in the current work are useful in guiding the design and fabrication of VGs for advancing their EDLC applications.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, College of Energy Engineering, Zhejiang University, Hangzhou, Zhejiang Province, 310027, China.

ABSTRACT
Vertically-oriented graphenes (VGs) are promising active materials for electric double layer capacitors (EDLCs) due to their unique morphological and structural features. This study, for the first time, reports the molecular dynamics (MD) simulations on aqueous NaCl electrolytes confined within VG channels with different surface charge densities and channel widths. Simulation results show that the accessibility of ions and the structure of EDLCs are determined by the ion type/size, surface charging, and VG channel width. For relatively narrow VG channels with the same width, the threshold charge density (to compensate the energy penalty for shedding hydration shell) and the dehydration rate of Cl(-) ions are larger than those of Na(+) ions. To achieve the highest ion concentration coefficient, the effective VG channel width should be between the crystal and hydration diameters of the ions. The results are further quantified and elucidated by calculating the electrolyte density profiles. The molecular insights obtained in the current work are useful in guiding the design and fabrication of VGs for advancing their EDLC applications.

No MeSH data available.


Related in: MedlinePlus

Distribution of electrolytes in VG channel of d = 12 Å.Area number density profiles of ions and water molecules within (a) negatively and (b) positively charged channels of d = 12 Å. The left Y axes present the density of atomic oxygen (black dashed lines) and hydrogen (red dashed lines) in water molecules. The right Y axes present the densities of Na+ ions (blue solid lines) and Cl− ions (green solid lines).
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f5: Distribution of electrolytes in VG channel of d = 12 Å.Area number density profiles of ions and water molecules within (a) negatively and (b) positively charged channels of d = 12 Å. The left Y axes present the density of atomic oxygen (black dashed lines) and hydrogen (red dashed lines) in water molecules. The right Y axes present the densities of Na+ ions (blue solid lines) and Cl− ions (green solid lines).

Mentions: Figure 5 shows the area number density profiles of ions and molecules within the VG channel of d = 12 Å. With an increasing charge density, the number of water molecules within VG channels varied a little (Supplementary Information, Fig. S3), but more molecules were adsorbed to the channel surface, presenting a multiple distinct layer structure. For the same absolute value of charge density, the orientation of water molecules within negatively and positively charged VG channels was found to be very different. This phenomenon was obvious especially at a relatively high surface charge density of ±15 μC cm−2, as shown in Fig. 5. In positively charged VG channels, the O and H peaks were almost at the same z position and the water molecules were probably in the plane parallel to the VG channel surface. In negatively charged VG channels, the O peak was sandwiched by two H peaks, which could be attributed to the channel-water electrostatic interactions (one of the H atoms could be attracted by the negatively charged carbon atoms). The distribution of Na+ and Cl− ions within VG channels also varied with an increasing surface charge density. Without charging, a small number of Na+ and Cl− ions were observed in the center of VG channels, consistent with the aforementioned results on ion concentration coefficient (see Fig. 2). With the increase of surface charge density, the EDLC structure was achieved by attracting counter-ions into VG channel and expelling the co-ions to the bulk region. The change of ion numbers as a function of the charge density can be found in Supplementary Information, Fig. S4. For the relatively high surface charge density of ±15 μC cm−2, obvious EDLC structures were constructed, consisting of two dense layers of ions contacting the charged surface. Cl− ions resided at a position embed inside the first H atoms of water molecules, while Na+ ions accumulated at a position depleted of water molecules. It suggests a favorable interaction between Cl− ions and H atoms of water molecules27. Meantime, there were a small number of co-ions found in the center of the negatively charged channels, probably due to the ion-ion correlations33. For wider channel of d = 16 Å, the structure of EDLCs (see Supplementary Information, Fig. S2) was similar to that of d = 12 Å.


Molecular Insights into Aqueous NaCl Electrolytes Confined within Vertically-oriented Graphenes.

Bo Z, Yang H, Zhang S, Yang J, Yan J, Cen K - Sci Rep (2015)

Distribution of electrolytes in VG channel of d = 12 Å.Area number density profiles of ions and water molecules within (a) negatively and (b) positively charged channels of d = 12 Å. The left Y axes present the density of atomic oxygen (black dashed lines) and hydrogen (red dashed lines) in water molecules. The right Y axes present the densities of Na+ ions (blue solid lines) and Cl− ions (green solid lines).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4589769&req=5

f5: Distribution of electrolytes in VG channel of d = 12 Å.Area number density profiles of ions and water molecules within (a) negatively and (b) positively charged channels of d = 12 Å. The left Y axes present the density of atomic oxygen (black dashed lines) and hydrogen (red dashed lines) in water molecules. The right Y axes present the densities of Na+ ions (blue solid lines) and Cl− ions (green solid lines).
Mentions: Figure 5 shows the area number density profiles of ions and molecules within the VG channel of d = 12 Å. With an increasing charge density, the number of water molecules within VG channels varied a little (Supplementary Information, Fig. S3), but more molecules were adsorbed to the channel surface, presenting a multiple distinct layer structure. For the same absolute value of charge density, the orientation of water molecules within negatively and positively charged VG channels was found to be very different. This phenomenon was obvious especially at a relatively high surface charge density of ±15 μC cm−2, as shown in Fig. 5. In positively charged VG channels, the O and H peaks were almost at the same z position and the water molecules were probably in the plane parallel to the VG channel surface. In negatively charged VG channels, the O peak was sandwiched by two H peaks, which could be attributed to the channel-water electrostatic interactions (one of the H atoms could be attracted by the negatively charged carbon atoms). The distribution of Na+ and Cl− ions within VG channels also varied with an increasing surface charge density. Without charging, a small number of Na+ and Cl− ions were observed in the center of VG channels, consistent with the aforementioned results on ion concentration coefficient (see Fig. 2). With the increase of surface charge density, the EDLC structure was achieved by attracting counter-ions into VG channel and expelling the co-ions to the bulk region. The change of ion numbers as a function of the charge density can be found in Supplementary Information, Fig. S4. For the relatively high surface charge density of ±15 μC cm−2, obvious EDLC structures were constructed, consisting of two dense layers of ions contacting the charged surface. Cl− ions resided at a position embed inside the first H atoms of water molecules, while Na+ ions accumulated at a position depleted of water molecules. It suggests a favorable interaction between Cl− ions and H atoms of water molecules27. Meantime, there were a small number of co-ions found in the center of the negatively charged channels, probably due to the ion-ion correlations33. For wider channel of d = 16 Å, the structure of EDLCs (see Supplementary Information, Fig. S2) was similar to that of d = 12 Å.

Bottom Line: This study, for the first time, reports the molecular dynamics (MD) simulations on aqueous NaCl electrolytes confined within VG channels with different surface charge densities and channel widths.The results are further quantified and elucidated by calculating the electrolyte density profiles.The molecular insights obtained in the current work are useful in guiding the design and fabrication of VGs for advancing their EDLC applications.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, College of Energy Engineering, Zhejiang University, Hangzhou, Zhejiang Province, 310027, China.

ABSTRACT
Vertically-oriented graphenes (VGs) are promising active materials for electric double layer capacitors (EDLCs) due to their unique morphological and structural features. This study, for the first time, reports the molecular dynamics (MD) simulations on aqueous NaCl electrolytes confined within VG channels with different surface charge densities and channel widths. Simulation results show that the accessibility of ions and the structure of EDLCs are determined by the ion type/size, surface charging, and VG channel width. For relatively narrow VG channels with the same width, the threshold charge density (to compensate the energy penalty for shedding hydration shell) and the dehydration rate of Cl(-) ions are larger than those of Na(+) ions. To achieve the highest ion concentration coefficient, the effective VG channel width should be between the crystal and hydration diameters of the ions. The results are further quantified and elucidated by calculating the electrolyte density profiles. The molecular insights obtained in the current work are useful in guiding the design and fabrication of VGs for advancing their EDLC applications.

No MeSH data available.


Related in: MedlinePlus