Prof. Enn Lust, TÜ keemia instituut
Prof. Alvo Aabloo, TÜ tehnoloogiainstituut
Prof. Ari Ivaska, Åbo Akademi, Soome
Electroactive polymers (EAPs) are a type of smart materials that convert electrical pulse energy into mechanical deformation or vice versa, and hence, can be utilized as actuators or sensors. EAPs are promising functional materials for engineering new and increasingly demanded actuation systems that are silent, mechanically compliant, lightweight, structurally simple, and easily scalable. Ionic type of EAPs that consist of electrolyte containing membrane between two conductive electrode layers are especially attractive for bio-robotic applications, and were studied in this work. It is commonly accepted that the increase of charge density within the electrodes leads to higher actuation performance of an EAP actuator. In this work, significant efforts have been put into exploring highly porous conductive materials for assembling EAP actuators with high specific surface area electrodes. The materials with highest potential for this purpose are highly porous amorphous carbons and carbide-derived carbons (CDC), commonly used in energy storage devices - supercapacitors. Promising less expensive alternatives are also carbon aerogels. In this thesis, carbide-derived carbon (CDC), coconut shell-based activated carbon, and carbon aerogel powders (activated and non-activated) were investigated as new alternative materials for application in electroactive polymer (EAP) actuator electrodes. The respective electrode materials were tested in five-layered actuator systems composed of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-TF)-impregnated Nafion membrane between two carbon electrode layers and gold foil on the surface. The electromechanical, electrochemical and mechanical characteristics of the prepared EAP actuators were examined and compared to the actuators based on RuO2 electrodes. The EAP actuator assembled with carbide-derived carbon electrodes produced the highest bending strain among the samples, up to 2.04% at ±2 V square-wave input, exceeding the strain of RuO2 electrodes by more than twice. The detailed analysis of the measured data indicates that the actuation performance of the EAPs is strongly affected by the porosity parameters and structural rigidity (degree of graphitization) of the carbon electrode materials. The effects of single-walled carbon nanotube (SWCNT) additives on the actuation performance were investigated in three-layered actuator systems composed of polymeric 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) electrolyte layer between two SWCNT/CDC electrode layers. The actuators were assembled with five different ratios of SWCNTs to CDC in the electrodes and their electromechanical, electrochemical and mechanical properties were comparatively analyzed. Analysis of the measured data demonstrates that the increase of the content of SWCNTs in the composite electrodes increased considerably the conductivity of electrodes and the stiffness (Young's modulus) of the actuators. The addition of SWCNTs into the electrodes improved significantly the bending strain output. At low frequency range (5 mHz-0.5 Hz), the SWCNT/CDC (50/50)-actuator showed by far the highest bending strain among the tested samples (up to 0.85%), which compared to the pure CDC electrodes with maximum strain of 0.35% is an improvement of more than two times. A strong co-effect of the specific capacitance and Young's modulus on the bending strain was observed at the lower frequency range.