Ionomeric Polymer-metal Composites Research Articles

Plasma treatment of nafion membranes: Understanding the trade-off between surface modification and electrochemical degradation

Nafion is a polymer commonly used in electrochemical devices due to its excellent ionic conductivity. In applications such as proton-exchange membranes for fuel cells (PEMFCs) and ionomeric polymer metal composites (IPMCs), the interfacial properties are critical to device performance. Several studies use plasma treatment to modify the Nafion surface aiming for adhesion improvement through nanoroughness, however, most of them neglect the effects of this treatment on the membrane chemical structure and electrochemical performance. This work comprehensively investigates the Nafion properties, evaluating chemical, morphological, and electrochemical modifications trhough relevant characterization methods. Atomic force (AFM) and scanning electron microscopies (SEM) showed increased nanoroughness with plasma treatment time. Also, Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses showed fluorine and sulfur abstraction, associated with side chain breakage. This effect was prejudicial to the water uptake and decreased the ionic conductivity, measured by Electrochemical Impedance Spectroscopy (EIS), from 140.8 to 1.0 μS cm−1 in the plasma-treated Nafion. Also, an applied electrochemical device was prepared and confirmed the ionic transport reduction using plasma-treated Nafion. This phenomenon is crucial for electrochemical devices, as fewer ionic channels remain on the treated membrane surface reducing water absorption, ionic conductivity, and charge density.

Sulfonated PEEK-based IPMC actuators: Exploring environmental influences

Ionomeric Polymer-Metal Composites (IPMCs) with a layered metal/polymer/metal structure are innovative biomimetic materials widely used in applications such as actuators, sensors, artificial muscles, and robotics due to their dynamic response to electrical stimuli. Nafion® is commonly used for these applications but is associated with high costs, low water retention, and poor performance at elevated temperatures. To address this, sulfonated poly(ether ether ketone) (SPEEK) is a promising and cost-effective alternative. This study focuses on evaluating the viability of SPEEK in IPMCs under varying environmental conditions, specifically a wide range of relative humidity (RH) levels (30%, 60%, and 90%) and two different counterions (H+ and Li+). FTIR tests confirmed the successful sulfonation of SPEEK, and sulfonation degrees (SD) of 44%, 72%, and 91% were determined by titration. Results indicate that increased SD leads to higher hydration levels, enhancing performance as actuators. Regarding electromechanical performance, SPEEK-based IPMCs demonstrated results comparable or superior to Nafion. Higher RH positively influenced electromechanical performance, increasing displacement and displacement rate values. Additionally, electrochemical/mechanical tests confirmed that, for the H+ counterion, SPEEK exhibited superior performance. In conclusion, SPEEK-based IPMCs, especially with H+ as the counterion, can potentially replace Nafion, offering a cost-effective alternative with favorable electromechanical performance under varying environmental conditions.

PI Controller for IPMC Actuators Based on Nafion®/PT Using Machine Vision for Feedback Response at Different Relative Humidities

Ionomeric Polymer-Metal Composites (IPMCs) are sandwich-like materials based in a polymeric membrane covered on both sides by metallic electrodes. Its operation mechanism consists of hydrated ions migration in response to an external electrical field generated between the electrodes, leading to a spatially nonuniform mass accumulation which causes the device to bend. Its performance as an actuator depends mainly on the environment's relative humidity and electrical stimuli. Consequently, IPMCs present variations in the electromechanical properties exhibiting nonlinearities and time-varying behaviors, limiting the major applications. For this reason, this paper investigated a PI controller performance to overcome these drawbacks and effectively control a Nafion-based IPMC-Li+ exposed to different relative humidities and electrical stimulis. The PI control system uses a camera with a machine vision application as a feedback sensor. Support instrumentation was developed to control the relative humidity, apply an electrical stimulus, and measure the electromechanical response. The pattern recognition algorithm implemented in the controller is efficient, with accuracy above 95%, making the feedback sensor reliable. Therefore, the PI controller was effective, stable, and capable of controlling and characterizing IPMC devices when relative humidity was above 60% at a low-frequency displacement and avoided the undesired back-relaxation phenomenon.

Electromechanical Evaluation of Ionomeric Polymer-Metal Composites Using Video Analysis

Abstract Ionomeric Polymer-Metal Composites (IPMC) are smart materials whose electromechanical behavior depends on the electrical stimulus intensity, membrane hydration level, and ionic migration. This paper investigates the effects of the voltage, relative humidity, and counterion type (Li+ and K+) on a Nafion-based IPMC performance. Instrumentation capable of applying an electrical stimulus and measuring the electromechanical response was developed. The ionic conductivity was obtained using Electrochemical Impedance Spectroscopy. Complementary SEM analyses were performed before and after actuation cycles. The IPMC performance improved when the electrical stimulus was 5.00 V, RH = 90%, and Li+ was used. The IPMC-Li sample is an excellent candidate to be used as an actuator since it exhibited fast actuation movement, considerable displacement, and no evident back-relaxation. However, its mechanical performance decreased over time because of a progressive increase in platinum electrode crack density and dehydration. The video analysis technique is an efficient, effective, and low-cost technique.

Electrochemomechanical Modeling and In Situ Measurement of Transition Metal Dichalcogenides-Based Electrochemical Actuators

Since working voltages are much higher than water electrolysis voltage, traditional ionic electroactive polymers (IEAPs) such as ionomeric polymer-metal composites and conjugated polymers still face the great challenges like back relaxation, leakage and evaporation of electrolyte. Newly developed transition metal dichalcogenides (TMDs)-based IEAP, which can be driven by the voltage as low as 0.3 V, becomes a promising candidate to overcome those challenges. Herein, an electrochemomechanical model, coupling ions intercalation and chemical reaction, is proposed for the first time to explain the mechanism of TMDs-based IEAPs. To further validate the model, molybdenum disulfide/Aluminum electrochemical actuators (EAs) are fabricated. Then chemical and mechanical performance of the EAs are recorded in situ during actuation. A good agreement is achieved by comparing theoretical and experimental results. A model study is performed to predict the effects of scan rate, working temperature and PH value of the electrolyte on curvature evolution. Results show that scan rate and temperature can hardly influence curvature amplitude while PH value of the electrolyte greatly affects curvature evolution. Moreover, response rate of EAs increases with scan rate. An optimal thickness ratio of 0.5 is also found for bilayered EAs. This study paves a new way for promoting the development of TMDs-based IEAPs.

Counter-ion and humidity effects on electromechanical properties of Nafion®/Pt composites

Ionomeric polymer/metal composites (IPMCs) are smart materials that deform in response to electrical stimuli and vice versa. Its electromechanical performance depends on several factors, such as the electrical stimulus, environment humidity, counterions, and the number of actuation cycles. For this reason, in this paper, the electromechanical response of Pt-Nafion® IPMC samples was evaluated using several counterion types (H+, Li+, Na+, K+ and Ionic Liquid) and relative humidities. Results showed that the electromechanical behaviour of the IPMC was strongly influenced by the counterion type and polymer matrix hydration level (RH). Blocking force, Electric Charge Storage and the Coulombic Efficiency of the devices increased with the reduction of counterion ionic radius and increase of the hydration level of the polymeric matrix. An atypical current response, associated with water electrolysis, was observed for samples incorporated with H+ counterion. The mechanical performance decreased with the number of cycles, showing a limited life cycle for the device. SEM images presented that Pt surface cracks density increases after several actuations, harming the performance of the IPMC. As the main conclusion, this work shows that the hydration level and the counterion type exert a great influence on IPMC electromechanical properties, being the hydration level more prevailing than counterion.

Effects of Block Ionomer Morphologies on the Electromechanical Performance of Ionomeric Polymer-Metal Composite Actuators

Ionomeric polymer-metal composites (IPMCs), which is a type of electroactive polymers, can generate a large and fast bending actuation under a low operating voltage (1~5V) (1). This superior electromechanical performance, combined with the inherent nature of polymer such as lightweight and softness, makes them attractive for biomimetic devices such as artificial muscles and soft robots (2). IPMCs have a tri-layer structure that consists of an ionomer film incorporating ions and water (or ionic liquids), and electrodes plated on both film surfaces (1-4). Upon applying a voltage, water and ions move toward the oppositely charged side leading to unsymmetrical polymer swelling. This change is restrained within the polymer’s nonionic matrix resulting in asymmetric stress that drives the bending motion. Advanced electromechanical performance of IPMCs relies on the ion and water transport ability and mechanical integrity of the used ionomer (3-5). In this work, the electromechanical performance of water-based IPMC actuators was examined as a function of the phase-separated morphology of a sulfonated penta-block ionomer (SBI). The SBI films---cast from a range of nonpolar/polar solvent blends---had a series of morphologies varied from poorly-connected ionic domains to well-connected lamellar/hexagonal structures. The poorly-connected structure led to a slow and small actuating response due to the small through-plane ion conductivity (1.5 mS/cm) and water permeability (1.3 × 10-6 mm2/(s MPa)). The SBI film with the lamellar/hexagonal structure however had an excessive water uptake (46wt. %) that greatly impaired the mechanical strength (53 MPa). The IPMC made from this film showed a faster and larger bending over the poorly-connected one but lost the advantage in terms of bending strain energy density. Interestingly, the best-performing IPMC is a SBI film possessing a lamella-like structure with its ionic domains perforated by nonionic polymer matrix. Its superior performance in terms of both bending displacement and strain energy density was attributed to the simultaneous improvements of ion and water transport (19.6 mS/cm and 3.1 × 10-5 mm2/(s MPa), respectively), and mechanical properties (125 MPa). Consequently, an optimal morphology exists for ideal electromechanical performance that is linked to molecule transport and ionomer mechanical properties. (1) Shahinpoor, M.; Kim, K. J. Ionic Polymer – Metal Composites : I .Fundemental. Smart Mater. Struct. 2001, 10, 819-833. (2) Shahinpoor, M.; Kim, K. J. Ionic Polymer–metal Composites: IV. Industrial and Medical Applications. Smart Mater. Struct. 2005, 14 (1), 197–214. (3) Nemat-Nasser, S. Micromechanics of Actuation of Ionic Polymer-Metal Composites. J. Appl. Phys. 2002, 92 (5), 2899–2915. (4) Asaka, K.; Oguro, K. Bending of Polyelectrolyte Membrane Platinum Composites by Electric Stimuli PartII. Response Kinetics. J. Electroanal. Chem. 2000, 480, 186–198. (5) Zhu, Z.; Asaka, K.; Chang, L.; Takagi, K.; Chen, H. Multiphysics of Ionic Polymer–metal Composite Actuator. J. Appl. Phys. 2013, 114 (8), 084902. Figure 1

Electro-chemical aspects of IPMCs within the framework of the theory of porous media

Ionomeric polymer–metal composites (IPMCs) consist of an ionomer with bound anionic groups and mobile counterions. They are plated with noble impermeable metal cover layers. By application of an electric voltage, a transport of the mobile ions towards the respective electrode occurs. Due to local electrostatic and ionic forces, a local deformation of the IPMC can be observed. Therefore IPMCs are promising candidates for electrochemical transducers. In the present research, the chemo-electro-mechanical behavior of IPMCs is described within the framework of the theory of porous media. First, the field equations are derived with respect to the second law of thermodynamics. Second, a reduced set of equations for the chemo-electric behavior is formulated and discretized by applying the finite element method. In the numerical investigations a parametric study of the time and space dependent behavior is carried out in order to quantify the influence of different material compositions. Based on this study, the characteristic response of IPMC to the application of an electric voltage can be predicted. Concluding, the obtained computational framework is an excellent tool for the design of electrochemical transducers.

Study of the Influence of the Hydration Level on the Electromechanical Behavior of Nafion Based Ionomeric Polymer-metal Composites Actuators

The ionomeric electroactive polymers-metal composites (IPMC) are materials that realize bending movements in response to an electrical stimulus and can be applied to the artificial muscles development. This paper investigated the influence of the hydration level on the electromechanical behavior of Nafion based IPMC. An experimental apparatus composed by a system to measure the bending force and to control the electrical stimulus and the relative humidity around the sample was developed. The main results indicated that samples with higher hydration level produce larger bending forces, but they show a more expressive back relaxation with lower time constant. A discussion about the results considering some models described in the literature conclude the paper.

Development of Artificial Muscles Based on Electroactive Ionomeric Polymer‐Metal Composites

This work contextualizes the research of materials that can be applied as artificial muscles. The main motivation of this research is the importance of the development of mechatronic systems for the replacement of traditional devices of actuation and motion based on rotational electrical motors by other devices that reproduce biological muscle movements. Electroactive polymers (EAPs) are materials that respond to electric stimuli with shape and/or dimension changes, and accomplish movements that are smooth enough to mimic biological muscles. Among EAPs, the ionomeric polymer-metal composites (IPMCs) are an interesting alternative to biomimetic devices due to large displacements when submitted to low applied voltage. This article presents a brief review of IPMCs, a sample preparation procedure, and some electromechanical experimental results. We also discuss the applicability of this technology in medical devices and as artificial muscles.

Electrode reactions in Cu–Pt coated ionic polymer actuators

Abstract Composite actuators consisting of sheets of the solid polymer electrolyte (similar to Nafion®) with Cu2+ counter ions inserted and coated with platinum and copper metal layers (so-called ionomeric polymer–metal composites; IPMCs) have been synthesised and their electromechanical performance upon actuation has been monitored. Resistance measurements on the electrodes show that the electrical conductivity of the membranes metal surface increases on the cathode side during the actuation process, contradictory to the situation when Cu is absent from the metal coating. This phenomenon is explained by the subsequent reduction of Cu2+ ions on the cathode upon actuation; Cu layer growth in this side prevents it from cracking and decreases its electrode resistance. The phenomenon opens up for longer lifetimes for Cu-based IPMCs. However, additional problems with Cu layer oxidation and Cu dendrite growth on the electrodes should be considered.