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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) Normalized time autocorrelation functions for the processes [A](t), where A denotes H3O+, H5O2+, and H3O+·(H2O)3, at λ = 10. (b) Time-dependent cross-correlation functions (in arbitrary units) characterizing the formation of different pairs of hydrated proton complexes at λ = 10.
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Figure 15: (a) Normalized time autocorrelation functions for the processes [A](t), where A denotes H3O+, H5O2+, and H3O+·(H2O)3, at λ = 10. (b) Time-dependent cross-correlation functions (in arbitrary units) characterizing the formation of different pairs of hydrated proton complexes at λ = 10.

Mentions: In Fig. 14 we present the relative content of hydrated proton complexes [A], A = H3O+, H5O2+, H3O+·(H2O)3, as a function of simulation time. The time autocorrelation functions, C(A;t), calculated for these processes and the time-dependent cross-correlation functions, C(A,B;t), characterizing the correlation between three different pairs of the same complexes are shown in Fig. 15. It is notable that the C(A;t) correlation functions exhibit an exponential decay at very short time (≈10 fs). The estimated relaxation time τh associated with the formation of hydronium ions is greater than the relaxation time τZ found for Zundel ions, but is considerably less than the relaxation time τE found for Eigen ions. This result suggests that the proton exchange between hydronium and water molecules is a relatively fast process as compared to the formation of both hydronium and Eigen ions. An analogous conclusion can be drawn from the data shown in Fig. 15 for the cross-correlation functions. The reversible transition H3O+ + 3H2O ↔ H3O+·(H2O)3 is a strongly correlated process, as can be expected, while the mutual transformations of hydronioum/Zundel ions (reversible transition H3O+ + H2O ↔ H5O2+) and Zundel/Eigen ions (reversible transition H5O2+ + 2H2O ↔ H3O+·(H2O)3) are less correlated. Note that proton transmission becomes possible only when the surrounding water molecules rearrange at particular points in time to enable the Zundel cation and at other times the Eigen cation configuration.


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) Normalized time autocorrelation functions for the processes [A](t), where A denotes H3O+, H5O2+, and H3O+·(H2O)3, at λ = 10. (b) Time-dependent cross-correlation functions (in arbitrary units) characterizing the formation of different pairs of hydrated proton complexes at λ = 10.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

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

Figure 15: (a) Normalized time autocorrelation functions for the processes [A](t), where A denotes H3O+, H5O2+, and H3O+·(H2O)3, at λ = 10. (b) Time-dependent cross-correlation functions (in arbitrary units) characterizing the formation of different pairs of hydrated proton complexes at λ = 10.
Mentions: In Fig. 14 we present the relative content of hydrated proton complexes [A], A = H3O+, H5O2+, H3O+·(H2O)3, as a function of simulation time. The time autocorrelation functions, C(A;t), calculated for these processes and the time-dependent cross-correlation functions, C(A,B;t), characterizing the correlation between three different pairs of the same complexes are shown in Fig. 15. It is notable that the C(A;t) correlation functions exhibit an exponential decay at very short time (≈10 fs). The estimated relaxation time τh associated with the formation of hydronium ions is greater than the relaxation time τZ found for Zundel ions, but is considerably less than the relaxation time τE found for Eigen ions. This result suggests that the proton exchange between hydronium and water molecules is a relatively fast process as compared to the formation of both hydronium and Eigen ions. An analogous conclusion can be drawn from the data shown in Fig. 15 for the cross-correlation functions. The reversible transition H3O+ + 3H2O ↔ H3O+·(H2O)3 is a strongly correlated process, as can be expected, while the mutual transformations of hydronioum/Zundel ions (reversible transition H3O+ + H2O ↔ H5O2+) and Zundel/Eigen ions (reversible transition H5O2+ + 2H2O ↔ H3O+·(H2O)3) are less correlated. Note that proton transmission becomes possible only when the surrounding water molecules rearrange at particular points in time to enable the Zundel cation and at other times the Eigen cation configuration.

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.