expectations for CP structure, ionic mobility and electro-chemo-mechanical coupling.
The current work aims at answering the question, which are the advantages and the disadvantages of the concept of chemically synthesized electrodes and the combined synthesis method for the chosen applications.
1.6.1 Electrochemical synthesis on initially non-conducting surfaces
Tri-layer air-operated bending actuators were used as an application for the chemical-EC synthesis method with the goal to find the solution to the delami-nation problem using the chemical synthesis step in place of vacuum techniques, while retaining the advantages of EC prepared actuators. Resulting methods were intended to increase the technological suitability for commercialization and biocompatibility by avoiding the usage of noble metals and vacuum technologies. The prepared actuators had to be characterized mostly by means of EC methods. Comparison of the actuators of different designs and CPs was aimed at studying the implications of the developed method and identifying possible ways for further development. Expectations to the outcome of the research were:
finding possibilities and suitable chemical synthesis methods for the preparation of the EC synthesized working layers on a initially non-conducting membrane layer for air-operated metal-free actuators and electrolyte-operated linear actuators, considering also:
o different chemical synthesis methods for the preparation of the electrode layer;
o properties of the chemical synthesis substrates, such as hydrophobicity/
hydrophilicity, porosity/homogeneity (including SPE membranes), resistance to chemical synthesis media and oxidants;
o applicability of different CPs for chemical and EC layers and their combinations in order to control the structure and the properties of the actuators;
o need for EC synthesis in organic as well as aqueous media at lowered temperatures;
o availability of the materials, suitability for industrial production and the environmental impact;
o avoiding drawbacks in performance characteristics (comparing with alternative methods);
obtaining characterization results, allowing considerations of the chemical-EC synthesis method for other applications where chemical-EC coating is preferred on non-conducting surfaces.
1.6.2 Electrolyte-operated linear tri-layer actuators
CP actuators operate either in electrolyte or in air (also in vacuum). According to the application, the design could be radically different. In an electrolyte, freestanding single-layer CP films have been considered enough for linear actuation, and additional polymer membrane has not been considered necessary to operate in an electrolyte bath. In air application, a membrane in role of an ion reservoir is necessary.
Linear CP electrolyte-operated freestanding actuators are typically prepared as films, synthesized EC on a conducting substrate and peeled off afterwards.
To author’s best knowledge there have been no reports of truly stable actuation, either in terms (of lack) of creep or mechanical durability. Bending creep and the related issues are inherent to asymmetrical materials, deposited on an electrode and then removed, as the inner and outer layers are different in terms of structure and electroactivity. Linear creep behavior and ion mobility type depend solely on strain and internal stress of the CP material.
There are also the considerations of speed and force. For single-layer freestanding films, only the outer layer of the film is active due to the diffusion limitations of the ions. Thus, it would be beneficial to produce very thin actuators to ensure fast response. Such actuators would be fragile and difficult to handle. In case of thicker freestanding films and/or increased actuation speed – only the outer layer is accessible to ions, the inner part is passive and must be stretched (reminding the generally poor elasticity of CPs) by the outer layers.
The optimal linear actuator should be inherently symmetric, have the whole CP material accessible to ions and the thickness of the actuating CP layer should be related to the expected actuation speed and force output. Parts of the actuator, not participating in actuation, should have mechanical properties required by the intended application (typically better/different from those of CPs) and therefore tunable. The mechanical properties of this passive part should be able to suppress creep and retain the intended ion mobility type. The preparation method should ensure precise control over the mechanical, EC and ECMD properties. Development of the actuator, satisfying these requirements was intended as a proof of concept of the chemical-electrochemical synthesis method.
1.6.3 Large surface area conducting polymer electrodes with high ionic mobility
The research goal was to prepare large surface area CP hydrogels using the most classical and extensively studied components for chemical synthesis – polypyrrole, ammonium peroxydisulfate and sodium dodecylbenzenesulfonate for the traditional chemical synthesis. To date, for some reason, there have been no reports of successful hydrogel formation solely from such components.
Moreover, to author’s best knowledge, there are no reports that CP hydrogels have been used for carbonized aerogel formation.
Chemically synthesized large surface area electrodes were studied and characterized mainly in context of high power density CP energy storage devices with the goal to relieve the diffusion speed and the resulting power density limitation, inherent to CP energy storage applications. The following key properties were expected from results of the study:
preliminary study of the ranges of the suitable synthesis conditions and further EC modification options;
EC and environmental stability of the chemically synthesized CP hydrogels;
avoiding the disadvantages of alternative approaches;
environmental concerns, availability of the materials and simplicity (preferring single-component hydrogels) of the preparation methods;
acceptable control over the structure, chemical composition, mechanical, electrical and electro-chemo-mechanical properties.
While the main emphasis was on properties beneficial for energy storage devices, the prepared hydrogels and their EC modification options were cha-racterized in order to evaluate their feasibility and potential for other appli-cations requiring biocompatibility and exchange of the doping anions, expected for controlled drug delivery devices, in vivo and optionally anti-inflammatory electrodes and biocompatible anodes in microbial fuel cells. As a proof of concept, the prepared hydrogels were used as electrodes for the following EC synthesis inside of the hydrogel and as starting material for the high surface area PPy aerogels and carbonized aerogels.
2 EXPERIMENTAL
This chapter describes the used materials, preparation and measurement methods, technical set-ups and procedures for polymer synthesis and cha-racterization. Although several materials and methods for both chemical and EC synthesis step were used (detailed description in papers I–IV), only those leading to the most representative and reproducible results are described in the experimental section. Detailed information about the used reagents and equipment can be found in papers I–IV (referenced in text).
A combined chemical-EC synthesis method was used to prepare the actuators.
Working layers of the actuators were prepared in two steps. The chemically synthesized CP formed an electrode surface for the following EC deposition of the main actuating layer. The chemically synthesized electrode material remained mostly in the outer layer of the membrane, and the amount of chemi-cally synthesized CP decreased sharply towards the centre of the membrane.
The EC synthesis on this electrode resulted in the formation of tri-layer actuators.
All solutions, used in the preparation of the materials, EC and ECDM measurements were deaerated by ultrasonication and by saturation with Ar, where possible (no surfactants in solution) in order to reduce the amount of dissolved oxygen in solution, as at positive potentials the dissolved oxygen accelerates PPy degradation [191].
2.1.1 Substrates for chemical synthesis
Substrates were used as middle layers of the actuators in two different roles: as electrolyte storage and electronic separator layer for tri-layer air-operated bending actuators and as a mechanically supporting interlayer for electrolyte-operated tri-layer linear actuators.
Membranes used for the bending actuators were based on the PVdF polymer and PVdF-HFP co-polymers:
1. Commercial Millipore Immobilon-P (MPI, from product datasheet: PVdF, hydrophobic, thickness 125 µm, pore size 0.45 µm, porosity 70%);
2. Commercial Millipore Durapore (MPD, from product datasheet: PVdF, hydrophilic, thickness 110 µm, pore size 0.1 µm, porosity 70%);
3. In-house (IH) solid polymer gel electrolyte (SPE) membrane was prepared similarly to the procedure of Sugino et al. [127]. RTIL EMImBF4 (200 mg, 8 wt.%) and PVdF-HFP (200 mg, 8 wt.%) were dissolved in a mixture of MP (2 ml, 64 wt.%) and PC (500 mg, 20 wt.%). The mixture was stirred over night at 70 ºC and poured into mold. SPE film was obtained after the MP