Cage Effect-Induced Intermittent Dynamics Govern Proton Conductivity in Nafion Proton Exchange Membrane

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Proton transport in proton exchange membranes (PEMs) is vital for high-performance fuel cells (PEMFCs).
• Existing models of proton transport in Nafion, like Grotthuss and vehicle mechanisms, don't fully explain kinetic rate variations.
• Understanding microscopic dynamics is key to optimizing PEM design.

Purpose of the Study:

• To investigate the dynamic heterogeneity and intermittent behavior of hydrated protons during electric-field-driven diffusion in Nafion.
• To correlate proton mobility with the local chemical environment and dynamic characteristics.
• To elucidate the factors regulating proton jump probability under varying hydration and electric field conditions.

Main Methods:

• Molecular dynamics simulations were used to model electric-field-driven proton diffusion.
• Analysis focused on identifying intermittent dynamics, such as two-phase motion or exclusive cage rattling.
• The influence of the local chemical environment (water and fluorine content) on proton mobility was examined.

Main Results:

• Hydrated protons exhibit significant dynamic heterogeneity, alternating between cage rattling and cage jumping, or exclusively rattling.
• Cage jumps, facilitated by water-rich, fluorine-poor environments and cooperative motion of H9O4+ motifs, substantially increase proton mobility.
• Proton jump probability is synergistically controlled by hydration level and electric field strength, with their dominance shifting based on conditions.

Conclusions:

• Intermittent dynamics, particularly cage jumping, directly impact macroscopic proton conductivity in PEMs.
• The local chemical environment (cage composition) and external factors (hydration, electric field) are critical regulators of proton transport.
• Insights into these microscopic dynamics provide guidance for designing improved PEMs by controlling microstructural and dynamic properties.