Electrolytic expansion

In this section, the behaviour of the electrolytic expansion in conducting polymers, especially polyaniline and poly(o-methoxyaniline) (PMAN) are described, with discussion of the basic redox reaction of polyaniline, the dependence of the expansion ratios on oxidation levels, the kind of anions, strain, the pH of the electrolyte and anisotropy. 

Figure 8.2 Schematic diagrams for the measurement of electrolytic expansion along (a) the film length and (b) the thickness direction. WE, RE and CE are the working electrode, reference electrode and counter electrode, respectively.

The dependence of the electrolytic expansion rates in poly(o-methoxyaniline) film on the type of anions is shown in Figures 8.7a and 8.7b for pH = 0 and pH = 2, respectively. It should be noted that at pH = 0, the expansion rate scarcely depends on the kind of anion, whereas at pH = 2 the remarkable dependence is observed. The result indicates that at pH = 0 or pH < 1.5 in poly(o-methoxyaniline) the electrolytic expansion and contraction are certainly driven by the change of polymer conformation and/or the electrostatic repulsion. 

Figure 8.7 The dependency of electrolytic expansions in poly(o-methoxyaniline) films on the kind of anions at (a) pH 0, and (b) pH2. TSA is toluene sulfonic acid.

The electrolytic expansion for the thickness direction in polyaniline cast film [20] shows an extremely large expansion ratio of more than 25% as shown in Figure 8.10, and is comparable to that of natural muscles [6]. A similar result was also obtained in the cast film of poly(o-methoxyaniline) for the thickness direction. The large expansion ratio for the thickness direction is conjectured to relate to the condensation process of the cast film. It may be remarked that the evaporation of NMP solution results in shrinkage only in the thickness direction, but not in the area. Therefore, the cast film has more freedom to expand in the thickness direction than that parallel to the film surface. 

Figure 8.10 CV curve (upper) and electrolytic expansion (lower) ratio for the thickness...

Apart from polyaniline, other condncting polymers that are being studied for electrolytic expansion include polypyrrole [11, 15-17], poly(alkylthiophene) [26] and carbon nanotubes [5]. For example, electrochemically prepared polypyrrole films were used to study the qualitative movement of electrolytic expansion by fabricating a bimorph actuator. The movement of bending and stretching of the actuator was demonstrated in electrolyte solution [15]. Actuators fabricated by electrodeposition on gold-coated polyethylene films were studied [11] for the evaluation of expansion ratio and response time. Also, a microactuator of several tens of microns made from two layers of gold and... 

The fundamentals of electrolytic expansion in polyaniline films have been discussed. Ion insertion and exclusion by electrolytic oxidation and reduction are the primary mechanisms. However, it is also evident that the changes in molecular conformations, arising due to the delocalisation of 7t-electrons and the electrostatic repulsion between the polycations, are other mechanisms operating in a conducting polymer microactuator. By investigating the molecular structure and the higher order structure to optimise the electrolytic expansion, it should be possible to improve the expansion ratio and the force for practical usage. 

Catalyst Layer Cracking and Delamination Catalyst layers are typically sprayed, deposited, or spread onto the electrolyte from a viscous mixture. This mixture is then baked at an elevated temperature, which drives off volatile compounds in the catalyst mixture used to control mixture viscosity and dispersion. As a result, small fissures, or mudcracks are common in catalyst layers, as shown in Figure 6.32, with widths much greater than the average pore size in a continuous portion of the catalyst layer. Over time, and as a result of the electrolyte expansion and contraction with water content variation, these cracks can grow and lead to delamination or catalyst layer degradation. 

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