Friedrich Kaasik

MXene as a novel intercalation-type pseudocapacitive cathode and anode for capacitive deionization

In this proof-of-concept study, we introduce and demonstrate MXene as a novel type of intercalation electrode for desalination via capacitive deionization (CDI). Traditional CDI cells employ nanoporous carbon electrodes with significant pore volume to achieve a large desalination capacity via ion electrosorption. By contrast, MXene stores charge by ion intercalation between the sheets of its two-dimensional nanolamellar structure. By this virtue, it behaves as an ideal pseudocapacitor, that is, showing capacitive electric response while intercalating both anions and cations. We synthesized Ti3C2-MXene by the conventional process of etching ternary titanium aluminum carbide i.e., the MAX phase (Ti3AlC2) with hydrofluoric acid. The MXene material was cast directly onto the porous separator of the CDI cell without added binder, and exhibited very stable performance over 30 CDI cycles with an average salt adsorption capacity of 13 ± 2 mg g−1.

Ionic electroactive polymer artificial muscles in space applications

A large-scale effort was carried out to test the performance of seven types of ionic electroactive polymer (IEAP) actuators in space-hazardous environmental factors in laboratory conditions. The results substantiate that the IEAP materials are tolerant to long-term freezing and vacuum environments as well as ionizing Gamma-, X-ray, and UV radiation at the levels corresponding to low Earth orbit (LEO) conditions. The main aim of this material behaviour investigation is to understand and predict device service time for prolonged exposure to space environment.

Lifetime measurements of ionic electroactive polymer actuators

This article is focused on proposing a unified methodology for automating the measurement procedures of ionic electroactive polymer actuators. The proposed methodology and large-scale automation would make testing ionic electroactive polymer actuators less labor-intensive and allow analyzing many ionic electroactive polymer actuators simultaneously. Defining a clear framework for testing ionic electroactive polymer actuators performance and reliability would make the testing process reproducible and provide better comparison between ionic electroactive polymer actuators of either different or similar classes. Our methodology separates two types of degradation: degradation during operation and spontaneous self-degradation.

Carbon aerogel based electrode material for EAP actuators

In this work we report an actuator material, that consist of carbon aerogel, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and poly(vinylidene-co-hexafluoropropylene) (PVdF(HFP)). Actuators were made by using layer-by-layer casting method and they work as a bending actuators. Carbon aerogel is synthesized from 5- methylresorcinol, which is a waste product in oil-shale industry. It makes the material environmentally green. Carbon aerogels have a very low density and considerable specific surface area. It is generally understood that the large interfacial surface area of electrodes gives rise to better actuation performance; therefore, designing actuators with high specific surface area electrodes is of interest. The assembled three layer actuators require low voltage to operate and work steadily in open air due to non-volatile electrolyte. The electromechanical and electrical characteristics of prepared actuators were examined and compared to our previously reported actuators based on the carbide-derived carbon and activated carbon electrodes. The differences in actuation performance were analyzed in the context of pore characteristics and degree of graphitization of carbons. The gas sorption measurements were performed to characterize pore size distribution. These actuators show high strain, low back-relaxation and low power consumption and they are good for slow-response applications compared to carbon nanotube actuators.

Solvent and electrolyte effects in PPyDBS free standing films

Free standing conducting polymer films based on polypyrrole doped with dodecylbenzoesulfate (PPyDBS) are investigated in TBACF3SO3 (tetrabutylammonium trifluoromethanesulfonate) propylene carbonate (PC-Tf) followed in aqueous TMACl tetramethylammonium chloride (Aq-TM) with the aim to investigate actuation properties (anion or cation-driven actuation). Under isometric (constant force) conditions ECMD (electro-chemo-mechanical deformation) measurements are performed during cyclic voltammetric and chronoamperometric experiments. Electrolyte and solvent effects revealing that the actuation direction in propylene carbonate electrolyte changed from expansion at anodic potential to expansion at cathodic potentials during square wave potential steps. Finally if the PPyDBS film immersed in aqueous electrolyte the anion-driven actuation properties are maintenance. SEM measurements are implemented to reefer changes in film morphology and ion content (EDX, energy dispersive X-Ray) before and after actuation.

Self-sensing ionic electromechanically active actuator with patterned carbon electrodes

In comparison to other ionic electromechanically active polymers (ionic EAP), carbon-polymer composite (CPC) actuators are considered especially attractive due to possibility of producing completely metal-free devices. However, mechanical response of ionic EAP-s is, in addition to voltage and frequency, dependent on environmental variables such as humidity and temperature. Therefore, similarly to other EAPs, one of the major challenges lies in achieving controlled actuation of the CPC sample. Due to their size and added complexity, external feedback devices (e.g. laser displacement sensors and video cameras) tend to inhibit the application of micro-scale actuators. Hence, self-sensing EAP actuators – capable for simultaneous actuation and sensing – are often desired. A thin polyvinylidene fluoride-cohexafluoropropylene film with ionic liquid (EMIMBF4) was prepared and masked coincidently on opposite surfaces prior to spray painting carbide-derived carbon electrodes. The purpose of masking was to create different electrically insulated electrodes on the same surface of polymer in order to achieve separate sections for actuator and sensor on one piece of CPC material. Solution of electrode paint consisting of carbide-derived carbon, EMIMBF4 and dimethylacetamide was applied to the polymer film. After removing the masking tape, a completely metal-free CPC actuator with sophisticated electrode geometry was achieved to foster simultaneous sensing and actuation, i.e. self-sensing carbon-polymer actuator was created.

Long-term degradation of the ionic electroactive polymer actuators

The research is focused on lifetime and degradation of ionic electroactive polymer actuators (IEAP). The lifetime measurements were carried out using identical methodology upon the different IEAP types. The experiment conducted with large number of samples shows that two types of degradation have serious effect to the IEAPs: degradation during operation and spontaneous self-degradation. Additionally, two ways of occasional damage decrease their overall reliability. In the scope of the current paper we describe degradation of two different types of IEAP actuators: with carbonaceous electrodes and with conducting polymer electrodes. Nevertheless, the common evolutionary trends, rather than the comparative data analysis or formal statistics of all particular samples, are given. Analyzing the electromechanical and electrical impedances of the samples during their whole lifetime, we have found that observing the electric current gives adequate information about the degradation level of any IEAP actuator. Moreover, tracking this electrically measurable parameter enables detecting the occasional damage of an actuator.

Pulse-width-modulated charging of ionic and capacitive actuators

We report on using a pulse-width-modulated (PWM) signal for driving the ionic electroactive polymer (IEAP) actuators. The traditional approach for driving the IEAP actuators involves generation of complex analog signals. The proposed control method is substantially different: a digital PWM driving waveform is applied using an H-bridge driver. The two outputs of the H-bridge driver are switched between three states - they are either connected to the positive or negative power supply terminal, or disconnected, with a high-impedance output. An H-bridge can also be used for short-circuiting of the actuator, in turn improving the power-efficiency of the IEAP actuator. This control method is particularly beneficial in applying IEAP actuators in soft robotics.

Electromechanical characteristics of actuators based on carbide-derived carbon

An electromechanical actuator was prepared using non-ionic polymer, ionic liquid and carbide-derived carbon (CDC). Recently, simple layer-by-layer casting method for actuator production was discovered, using bucky gel mixture as the precursor of actuator electrode layers. In this paper we investigate carbide-derived carbon as a new alternative to carbon nanotubes to replace nanotubes in the electrode layer of the actuator. At the initial stage of the study, the ratio of nanoporous high surface TiC-derived carbon powder, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and polymer (PVdF(HFP)) was varied and each formed electrode was analyzed to find the optimal composition. The results revealed that the optimal component ratio for electrodes is: 35 wt% PVdF(HFP), 35 wt% EMIBF4 and 30 wt% CDC. The assembled three layer actuators were characterized by measuring blocking force, maximum strain, speed, power consumption and capacitance. The synthesized actuator showed very good force and capacitive characteristics and it is preferable for slow-response applications compared to actuators based on carbon nanotubes.

Soft shape-adaptive gripping device made from artificial muscle

We report on a multifunctional four-finger gripper for soft robotics, suitable for performing delicate manipulation tasks. The gripping device is comprised of separately driven gripping and lifting mechanisms, both made from a separate single piece of smart material - ionic capacitive laminate (ICL) also known as artificial muscle. Compared to other similar devices the relatively high force output of the ICL material allows one to construct a device able to grab and lift objects exceeding multiple times its own weight. Due to flexible design of ICL grips, the device is able to adapt the complex shapes of different objects and allows grasping single or multiple objects simultaneously without damage. The performance of the gripper is evaluated in two different configurations: a) the ultimate grasping strength of the gripping hand; and b) the maximum lifting force of the lifting actuator. The ICL is composed of three main layers: a porous membrane consisting of non-ionic polymer poly(vinylidene fluoride-co-hexafluoropropene) (PVdF-HFP), ionic liquid 1-ethyl-3-methylimidazolium trifluoromethane-sulfonate (EMITFS), and a reinforcing layer of woven fiberglass cloth. Both sides of the membrane are coated with a carbonaceous electrode. The electrodes are additionally covered with thin gold layers, serving as current collectors. Device made of this material operates silently, requires low driving voltage (<3 V), and is suitable for performing tasks in open air environment.