Effects of cation on electrical responses of ionic polymer-metal composite sensors at various ambient humidities

Abstract

In this study, we investigated the effects of various cations on the electrical responses of ionic polymer–metal composite (IPMC) sensors at various ambient humidities. Four typical Au–Nafion IPMC samples were prepared with H+, Li+, Na+, and K+ cations. The voltage and current responses of the IPMCs were investigated under static and dynamic bending displacements. The orders of the voltage and current amplitudes were generally Li+ > Na+ > K+ > H+ and depended on the cation transport properties and the water content. The static voltage response first increased to a peak and then slowly decreased to a steady state. A negative steady-state voltage was initially observed for the IPMC with H+ cations under near saturation conditions. The voltage amplitude increased monotonously with increasing frequency from 0.1 to 10 Hz at a high relative humidity (RH, ∼90%), first increased and then decreased at moderate humidity (RH, ∼50%), and decreased continuously at low humidity (RH, ∼20%). The static current response first rapidly increased to a peak and then quickly decayed. During current decay, free oscillation decay occurred at high humidity and attenuated with decreasing humidity. This was confirmed to be the result of cation movement in the IPMC. There are three necessary conditions for oscillation: sufficient migrated cations, high cation mobility, and high stiffness of the polymer network. For the dynamic current response, the amplitude increased with increasing frequency (0.1–10 Hz) and showed good linearity. The underlying physics, mainly involving cation forward migration and back diffusion caused by mechano-chemo-electrical coupling, was clarified.