Proton diffusion in the hexafluorophosphoric acid clathrate hydrate

Hexafluorophosphoric acid clathrate hydrates are known for high proton conductivity at room temperature. Prior research has shown that these materials can exhibit proton conductivities up to 0.1 S/cm. However, the mechanisms behind this conductivity remain unclear. Established methods like neutron scattering and NMR have been used to study proton movement in similar systems. This gap motivated a closer look at how protons move within the structure of hexafluorophosphoric acid hydrates. No prior work had resolved the specific role of structural transitions in proton diffusion. The distinction between long-range and short-range proton motion is still debated. This uncertainty drove the use of advanced experimental techniques to isolate the diffusion process. Understanding proton transport in these materials could help in designing better proton conductors.

The aim of this study was to investigate the proton diffusion mechanisms in hexafluorophosphoric acid clathrate hydrates. The specific problem addressed is the lack of clarity on how protons move within the material across different structural phases. This uncertainty has hindered the full understanding of proton conduction in these hydrates. The study focused on the structural transition between type I and type VII clathrate phases. The motivation was to determine whether proton diffusion occurs in each phase and how it is affected by structural changes. The researchers sought to use experimental methods to distinguish between long-range and short-range proton movement. This work aims to clarify the microscopic mechanism of proton conduction in this hydrate system.

The study used incoherent quasi-elastic neutron scattering (QENS) and proton pulsed-field-gradient nuclear magnetic resonance ((1)H PFG-NMR) to analyze proton diffusion. These methods allowed the researchers to observe proton movement at different time scales. The material was studied in two structural phases: type I (SI) and type VII (SVII). The SI phase was examined at low temperatures, while the SVII phase was studied above 230 K. The researchers measured proton diffusion rates in each phase using these techniques. The QENS data were used to model the proton movement with a Chudley-Elliot jump diffusion model. The (1)H PFG-NMR provided additional data on the long-range proton movement. These combined methods enabled a detailed analysis of proton transport mechanisms.

The study found that no long-range proton diffusion occurs in the type I (SI) phase at low temperatures. The diffusion rate in this phase is below the detection limit of 10(-7) cm(2)·s(-1). In contrast, the type VII (SVII) phase shows significant long-range proton diffusion. At 275 K, the diffusion coefficient is 3.85 × 10(-6) cm(2)·s(-1) with an activation energy of 0.19 ± 0.04 eV. The QENS data suggest that protons jump between water-oxygen sites. The jump distance was modeled as 2.79 ± 0.17 Å using the Chudley-Elliot model. The proton movement follows a Grotthus mechanism in the SVII phase. Additional water molecules in the SVII structure act as barriers to proton diffusion.

The authors conclude that long-range proton diffusion occurs only in the type VII (SVII) phase of the hydrate. The SI phase does not support significant proton movement at low temperatures. The SVII phase enables proton conduction via a Grotthus mechanism. The jump distance between water-oxygen sites is approximately 2.79 Å. The presence of additional water molecules in the SVII structure reduces proton diffusivity. These findings suggest that structural changes influence proton transport in the hydrate. The study provides a microscopic model for proton movement in this system. The results confirm the role of structural defects in limiting proton conductivity.