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Large-scale atomistic and quantum-mechanical simulations of a Nafion membrane: Morphology, proton solvation and charge transport.

Komarov PV, Khalatur PG, Khokhlov AR - Beilstein J Nanotechnol (2013)

Bottom Line: For the water/Nafion systems containing more than 4 million atoms, it is found that the observed microphase-segregated morphology can be classified as bicontinuous: both majority (hydrophobic) and minority (hydrophilic) subphases are 3D continuous and organized in an irregular ordered pattern, which is largely similar to that known for a bicontinuous double-diamond structure.A thermodynamic decomposition of the potential of mean force and the calculated spectral densities of the hindered translational motions of cations reveal that ion association observed with decreasing temperature is largely an entropic effect related to the loss of low-frequency modes.The extensive 120 ps-long density functional theory (DFT)-based simulations of charge migration in the 1200-atom model of the nanochannel consisting of Nafion chains and water molecules allowed us to observe the bimodality of the van Hove autocorrelation function, which provides the direct evidence of the Grotthuss bond-exchange (hopping) mechanism as a significant contributor to the proton conductivity.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Organoelement Compounds, RAS, Moscow 119991, Russia ; Department of Theoretical Physics, Tver State University, Tver 170002, Russia.

ABSTRACT
Atomistic and first-principles molecular dynamics simulations are employed to investigate the structure formation in a hydrated Nafion membrane and the solvation and transport of protons in the water channel of the membrane. For the water/Nafion systems containing more than 4 million atoms, it is found that the observed microphase-segregated morphology can be classified as bicontinuous: both majority (hydrophobic) and minority (hydrophilic) subphases are 3D continuous and organized in an irregular ordered pattern, which is largely similar to that known for a bicontinuous double-diamond structure. The characteristic size of the connected hydrophilic channels is about 25-50 Å, depending on the water content. A thermodynamic decomposition of the potential of mean force and the calculated spectral densities of the hindered translational motions of cations reveal that ion association observed with decreasing temperature is largely an entropic effect related to the loss of low-frequency modes. Based on the results from the atomistic simulation of the morphology of Nafion, we developed a realistic model of ion-conducting hydrophilic channel within the Nafion membrane and studied it with quantum molecular dynamics. The extensive 120 ps-long density functional theory (DFT)-based simulations of charge migration in the 1200-atom model of the nanochannel consisting of Nafion chains and water molecules allowed us to observe the bimodality of the van Hove autocorrelation function, which provides the direct evidence of the Grotthuss bond-exchange (hopping) mechanism as a significant contributor to the proton conductivity.

No MeSH data available.


Related in: MedlinePlus

The potential of mean force, W+–(r), and the energetic and entropic contributions, ΔU and –TΔS, to W+–(r) as a function of the SO3−–H3O+ separation at T = 300 K.
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Figure 7: The potential of mean force, W+–(r), and the energetic and entropic contributions, ΔU and –TΔS, to W+–(r) as a function of the SO3−–H3O+ separation at T = 300 K.

Mentions: From the analysis of PCFs we concluded that the ion association is seen to decrease with increasing temperature but a quite strong ion pairing is observed even at T = 500 K. On the other hand, no complete clustering is visible even at the lowest temperature considered. It was found that the W+–(r) function demonstrates a rather typical behavior well known for electrolyte solutions: there are one or two deep minima at the distance corresponding to associated sulfonate/hydronium ion pairs SO3−·H3O+ and solvent-separated ion pairs as well as a shallow third minimum at longer distances, probably due to a second solvation shell for the ions (Fig. 7). As the temperature is subsequently increased, the minimum for the contact ion pairs and solvent-separated ion pairs becomes deeper. More importantly, the main contribution to the free energy of contact ion pairs seems to be dominated by the entropy gain, not the potential energy contribution. To demonstrate this, we performed a thermodynamic decomposition of the potential of mean force.


Large-scale atomistic and quantum-mechanical simulations of a Nafion membrane: Morphology, proton solvation and charge transport.

Komarov PV, Khalatur PG, Khokhlov AR - Beilstein J Nanotechnol (2013)

The potential of mean force, W+–(r), and the energetic and entropic contributions, ΔU and –TΔS, to W+–(r) as a function of the SO3−–H3O+ separation at T = 300 K.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3817934&req=5

Figure 7: The potential of mean force, W+–(r), and the energetic and entropic contributions, ΔU and –TΔS, to W+–(r) as a function of the SO3−–H3O+ separation at T = 300 K.
Mentions: From the analysis of PCFs we concluded that the ion association is seen to decrease with increasing temperature but a quite strong ion pairing is observed even at T = 500 K. On the other hand, no complete clustering is visible even at the lowest temperature considered. It was found that the W+–(r) function demonstrates a rather typical behavior well known for electrolyte solutions: there are one or two deep minima at the distance corresponding to associated sulfonate/hydronium ion pairs SO3−·H3O+ and solvent-separated ion pairs as well as a shallow third minimum at longer distances, probably due to a second solvation shell for the ions (Fig. 7). As the temperature is subsequently increased, the minimum for the contact ion pairs and solvent-separated ion pairs becomes deeper. More importantly, the main contribution to the free energy of contact ion pairs seems to be dominated by the entropy gain, not the potential energy contribution. To demonstrate this, we performed a thermodynamic decomposition of the potential of mean force.

Bottom Line: For the water/Nafion systems containing more than 4 million atoms, it is found that the observed microphase-segregated morphology can be classified as bicontinuous: both majority (hydrophobic) and minority (hydrophilic) subphases are 3D continuous and organized in an irregular ordered pattern, which is largely similar to that known for a bicontinuous double-diamond structure.A thermodynamic decomposition of the potential of mean force and the calculated spectral densities of the hindered translational motions of cations reveal that ion association observed with decreasing temperature is largely an entropic effect related to the loss of low-frequency modes.The extensive 120 ps-long density functional theory (DFT)-based simulations of charge migration in the 1200-atom model of the nanochannel consisting of Nafion chains and water molecules allowed us to observe the bimodality of the van Hove autocorrelation function, which provides the direct evidence of the Grotthuss bond-exchange (hopping) mechanism as a significant contributor to the proton conductivity.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Organoelement Compounds, RAS, Moscow 119991, Russia ; Department of Theoretical Physics, Tver State University, Tver 170002, Russia.

ABSTRACT
Atomistic and first-principles molecular dynamics simulations are employed to investigate the structure formation in a hydrated Nafion membrane and the solvation and transport of protons in the water channel of the membrane. For the water/Nafion systems containing more than 4 million atoms, it is found that the observed microphase-segregated morphology can be classified as bicontinuous: both majority (hydrophobic) and minority (hydrophilic) subphases are 3D continuous and organized in an irregular ordered pattern, which is largely similar to that known for a bicontinuous double-diamond structure. The characteristic size of the connected hydrophilic channels is about 25-50 Å, depending on the water content. A thermodynamic decomposition of the potential of mean force and the calculated spectral densities of the hindered translational motions of cations reveal that ion association observed with decreasing temperature is largely an entropic effect related to the loss of low-frequency modes. Based on the results from the atomistic simulation of the morphology of Nafion, we developed a realistic model of ion-conducting hydrophilic channel within the Nafion membrane and studied it with quantum molecular dynamics. The extensive 120 ps-long density functional theory (DFT)-based simulations of charge migration in the 1200-atom model of the nanochannel consisting of Nafion chains and water molecules allowed us to observe the bimodality of the van Hove autocorrelation function, which provides the direct evidence of the Grotthuss bond-exchange (hopping) mechanism as a significant contributor to the proton conductivity.

No MeSH data available.


Related in: MedlinePlus