Chemo-electric characterization and modeling of the high surface area electrodes in ionic polymer transducers

Abstract

Ionomeric polymer transducers have received considerable attention in the past ten years due to their ability to generate large bending strain and moderate stress at low applied voltages. Ionic polymer transducers consist of an ionomer, usually Nafion, sandwiched between two electrically conductive electrodes. Recently, a novel fabrication technique denoted as the direct assembly process (DAP) enabled controlled electrode architecture in ionic polymer transducers. A DAP transducer consists of two high surface area electrodes made of uniform distributed particles sandwiching an ionomer membrane. In this paper theoretical investigations as well as experimental verifications are performed. The model consists of a convection-diffusion equation describing the chemical field as well as a Poisson equation describing the electrical field. This modeling technique is modified to capture the chemo-electric behavior in the high surface area electrodes of a DAP fabricated transducer. The model assumes highly conductive particles randomly distributed in the electrode area. This is the first electro-chemical modeling account of high surface area electrodes in ionic polymer transducers. Traditionally, these kinds of electrodes were simulated with boundary conditions representing flat electrodes with a large dielectric permittivity at the polymer boundary. In the experimental section, several transducers are fabricated using the DAP process on Nafion 117 membranes. The architecture of the high surface area electrodes in these samples is varied. The concentration of the spherical gold particles is varied from 30 vol% up to 60 vol%, while the overall thickness of the electrode is varied from 10 &mgr;m up to 40 &mgr;m micrometer at a fixed concentration. The flux and charge accumulation in the materials are measured experimentally and compared to the results of the numerical simulations. Finally, the high surface area electrodes are compared to flat gold electrodes.