Manufacture and Characterization of Ionic Polymer Transducers Employing Non-Precious Metal Electrodes
Ionic polymer membranes are commonly used in fuel cell power generation, water electrolysis and desalinization, chlorine generation, and other niche applications. Since the early 1990s ionic polymer membranes have also shown promise as distributed electromechanical actuators and sensors. The cost of these materials is very high because of the expensive noble metals that are used as the electrodes in these applications, however. Currently, high cost of these devices has prevented them from experiencing widespread use. The goal of the current research project is to study new methods of plating metal electrodes onto ionic polymer membranes in order to reduce the cost of these materials and open the door for potential industrial, aerospace, and biomedical applications.
At this time ionic polymer actuators are only made using gold or platinum as the electrode in a lengthy and labor-intensive process. The current research focuses on using less costly metals and revising the metal deposition process. Several new methods allowing for faster deposition of metals onto ionic polymer membranes are developed and evaluated including sputter-coating, electroless plating, and impregnation/reduction. Using these methods, metal electrodes have been plated onto ionic polymer membranes in processes resulting in a purely surface deposition and in processes resulting in interpenetration of the metal into the polymer. This work shows that electromechanical coupling is present with all of these processes, although results indicate that interpenetration of the electrode is important for good adhesion of the metal and good performance of the transducers.
Also studied were different metals; X-ray photoelectron spectroscopy (XPS) testing shows that the use of non-noble metals as the electrodes results in oxidation of the metal and corresponding loss of performance in the actuator. Noble metals are found to not experience the oxidation problem. Further work shows that non-noble metals can be effectively employed as electrodes if alloyed with noble metals by using a co-reduction technique. Also studied is the use of protective coatings of noble metal to stabilize the non-noble metal electrodes. Using these approaches, a new plating method is developed and the stability of the electrodes made using this method is studied. These results indicate that samples made using this new process may be actauted continuously for over 150,000 cycles with very little degredation in their performance.
Using this new plating method, ionic polymer membrane transducers can be made in less than five hours. Characterization of these new devices shows that they have a mass energy density of 4-20 mJ/kg in the cantilevered mode. This compares well with a baseline material, which is found to have a mass energy density of 3-12 mJ/kg. Composition and morphology of the electrodes made using the new method are investigated using scanning electron microscopy (SEM) and the density and tensile modulus are measured. The density of the new material is found to be approximately 2100 kg/m^3 as compared to about 3200 kg/m^3 for the baseline material. Also, the tensile elastic modulus of the new material is about 55 MPa, or roughly one fourth of the tensile modulus of the baseline material (about 190 MPa). These results indicate that the new materials contain much less noble metal in the electrodes than the baseline material. The sensitivity of these devices has also been quantified and compared to the baseline. Results indicate that the new materials have a sensitivity on the order of 0.1-0.3 uA/mm/s whereas similarly sized samples of the baseline material typically have sensitivities on the order of 0.2-0.8 uA/mm/s. The most important conclusion of this work is that ionic polymer membrane transducers can be made using much less noble metal in the electrode than previously believed without sacrificing the performance of these devices.