Structural characteristics of hydrated protons in the conductive channels: effects of confinement and fluorination studied by molecular dynamics simulation

Abstract

The relationship between the proton conductive channel and the hydrated proton structure is of significant importance for understanding the deformed hydrogen bonding network of the confined protons which matches the nanochannel. In general, the structure of hydrated protons in the nanochannel of the proton exchange membrane is affected by several factors. To investigate the independent effect of each factor, it is necessary to eliminate the interference of other factors. In this paper, a one-dimensional carbon nanotube decorated with fluorine was built to investigate the independent effects of nanoscale confinement and fluorination on the structural properties of hydrated protons in the nanochannel using classical molecular dynamics simulation. In order to characterize the structure of hydrated protons confined in the channel, the hydrogen bonding interaction between water and the hydrated protons has been studied according to suitable hydrogen bond criteria. The hydrogen bond criteria were proposed based on the radial distribution function, angle distribution and pair-potential energy distribution. It was found that fluorination leads to an ordered hydrogen bonding structure of the hydrated protons near the channel surface, and confinement weakens the formation of the bifurcated hydrogen bonds in the radial direction. Besides, fluorination lowers the free energy barrier of hydronium along the nanochannel, but slightly increases the barrier for water. This leads to disintegration of the sequential hydrogen bond network in the fluorinated CNTs with small size. In the fluorinated CNTs with large diameter, the lower degree of confinement produces a spiral-like sequential hydrogen bond network with few bifurcated hydrogen bonds in the central region. This structure might promote unidirectional proton transfer along the channel without random movement. This study provides the cooperative effect of confinement dimension and fluorination on the structure and hydrogen bonding of the slightly acidic water in the nanoscale channel.

Structural Characteristics of Hydrated Protons in Ion Conductive Channels: Synergistic Effect of the Sulfonate Group and Fluorine Studied by Molecular Dynamics Simulation

“…Compared with the nonfluorinated CNTs, the overall free energy profile of the hydronium ion in each fluorinated CNT is lowered by the functionalized fluorine atoms. This is in accordance with our previous results 19 that fluorination lowers the potential barrier for the hydronium ion along the fluorinated CNT. Especially for the N1, N2, and N3 systems, the depth of the energy minimum is decreased by the modified fluorine atoms.…”

“…It is necessary to employ classical molecular dynamics (CMD) simulation to have a longer observation of the nanoscale properties of the hydrogen bond network and the dynamic motions of water and proton in the PCC. Our recent CMD studies 19,20 show that confinement and fluorination have a cooperative effect on the structure of the hydrogen bond network in the fluorinated CNT with fixed size. An appropriate combination of confinement and fluorination produces a spiral-like hydrogen bond network with few bifurcated hydrogen bonds in the central region, which is beneficial to unidirectional proton transfer along the channel without random movement.…”

“…It has been reported that the hydrogen bond network inside the fully fluorinated CNT(10, 10) is beneficial to proton transfer. 19 Herein, the PCC backbone is modeled with CNT(10, 10), which is fixed during the simulation run in order to maintain the confinement effect. The CNTs are functionalized by one or two sulfonate groups with different spacings to compare the interplay between sulfonate groups.…”

OH− and H3O+ Diffusion in Model AEMs and PEMs at Low Hydration: Insights from Ab Initio Molecular Dynamics

“…However, enhancing hydroxide ion conductivity and membrane stability remains a key hurdle to realizing the potential of AEM fuel cells [ 2 , 3 , 5 ]. Compared to AEMs, fuel cell-based proton exchange membranes (PEMs) have received far more attention over the last decade due to their promise in technologies for clean and efficient power generation [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. The morphology, structure, and diffusion mechanism of hydronium ions in these devices have been investigated under a variety of environmental conditions.…”

Water Layering Affects Hydroxide Diffusion in Functionalized Nanoconfined Environments

“…Among these devices, fuel-cell-based proton exchange membranes (PEMs) have been studied extensively over the past decade. − The morphology, structure, and diffusion mechanism of hydronium ions in these devices have been investigated under a variety of confined environments. In contrast, hydroxide ion diffusion in fuel-cell-based anion exchange membranes (AEMs) has received far less attention, even though AEMs are considered to be a low-cost, clean-energy technology.…”