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Time-resolved mapping of water diffusion coefficients in a working soft actuator device
Leila Naji1, John A Chudek, Richard T Baker
1EaStChem, School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK.
The Journal of Physical Chemistry. B
|July 18, 2008
Summary
Diffusion-weighted imaging revealed how water diffusion in Nafion changes within an ionic polymer-metal composite (IPMC) actuator. Platinum electrode presence and applied voltage significantly alter water diffusion patterns in the Nafion material.
Area of Science:
- Materials Science
- Electrochemistry
- Polymer Science
Background:
- Nafion is a perfluorinated sulfonic acid polymer widely used in electrochemical applications.
- Ionic Polymer-Metal Composites (IPMCs) are soft actuators whose performance depends on ion and water transport within the polymer matrix.
- Understanding water diffusion is crucial for optimizing IPMC actuator design and function.
Purpose of the Study:
- To spatially map water diffusion coefficients (D) in bare Nafion and in an IPMC actuator.
- To investigate the influence of platinum (Pt) electrodes on water diffusion within the Nafion structure.
- To examine the effect of an applied DC potential on water diffusion distribution in the IPMC.
Main Methods:
- Diffusion-weighted imaging (DWI) was used to measure the diffusion coefficient of water.
- DWI was applied to bare, water-soaked, Li(+)-exchanged Nafion and a Pt-impregnated IPMC actuator.
- Water diffusion was measured in two orthogonal directions (Dx and Dz) with and without an applied 3 V DC potential.
Main Results:
- Bare Nafion exhibited uniform water diffusion (D ≈ 6 x 10⁻¹⁰ m²/s) in both directions.
- In the IPMC, water diffusion was higher at the Pt-electrode surfaces compared to the center, particularly in the through-thickness direction (Dz).
- During electrochemical measurements, higher D values were observed at the cathode (up to 8 x 10⁻¹⁰ m²/s) and lower values at the anode (from 1 x 10⁻¹⁰ m²/s).
Conclusions:
- The impregnation of Pt electrodes significantly alters the polymer structure and water diffusion within Nafion.
- The observed diffusion gradients in the IPMC are attributed to the influence of the Pt electrodes on the Nafion nanostructure.
- Electrochemical migration of Li(H₂O)ₓ⁺ species towards the cathode under applied potential drives changes in Nafion nanostructure and water diffusion.
