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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.


(a) Pair correlation functions, gOH(r), for the oxygen atoms of the SO3 groups and any proton, at λ = 5 and 10. (b) Pair correlation functions, gOO(r), for the oxygen atoms of H2O and proton–water complexes, at λ = 5 and 10.
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Figure 12: (a) Pair correlation functions, gOH(r), for the oxygen atoms of the SO3 groups and any proton, at λ = 5 and 10. (b) Pair correlation functions, gOO(r), for the oxygen atoms of H2O and proton–water complexes, at λ = 5 and 10.

Mentions: Additionally we analyzed pair correlation functions (PCFs). Fig. 12 shows PCF gOH(r), where O indicates the oxygen atom belonging to the SO3 groups and H denotes any hydrogen. At the hydration level λ = 5, the first sharp peak located at r ≈ 1.01 Å corresponds to the O–H bond distance in non-dissociated SO3H. This means that at low water loading, not all SO3H groups are ionized. At the same time, it should be stressed that perfluorosulfonic acids require only three water molecules to exhibit spontaneous proton dissociation [57]. At the higher hydration level, λ = 10, practically all protons are dissociated and this "valence" peak is not visible. The main peak in gOH(r) is associated with the first coordination shell formed by water molecules and positively charged ions around the SO3 groups, giving an average first solvation shell size of approximately 3.5 water molecules.


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)

(a) Pair correlation functions, gOH(r), for the oxygen atoms of the SO3 groups and any proton, at λ = 5 and 10. (b) Pair correlation functions, gOO(r), for the oxygen atoms of H2O and proton–water complexes, at λ = 5 and 10.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

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

Figure 12: (a) Pair correlation functions, gOH(r), for the oxygen atoms of the SO3 groups and any proton, at λ = 5 and 10. (b) Pair correlation functions, gOO(r), for the oxygen atoms of H2O and proton–water complexes, at λ = 5 and 10.
Mentions: Additionally we analyzed pair correlation functions (PCFs). Fig. 12 shows PCF gOH(r), where O indicates the oxygen atom belonging to the SO3 groups and H denotes any hydrogen. At the hydration level λ = 5, the first sharp peak located at r ≈ 1.01 Å corresponds to the O–H bond distance in non-dissociated SO3H. This means that at low water loading, not all SO3H groups are ionized. At the same time, it should be stressed that perfluorosulfonic acids require only three water molecules to exhibit spontaneous proton dissociation [57]. At the higher hydration level, λ = 10, practically all protons are dissociated and this "valence" peak is not visible. The main peak in gOH(r) is associated with the first coordination shell formed by water molecules and positively charged ions around the SO3 groups, giving an average first solvation shell size of approximately 3.5 water molecules.

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.