Nafion-based ionomeric polymer/metal composites operating in the air: theoretical and electrochemical analysis

Abstract

Nafion/Pt-based ionic polymer metal composites can be considered smart materials with characteristics that allow it to be used as soft actuators. Its working mechanism is based on solvated counterions moving through hydrophilic sites of the polymer membrane, making it an electrochemical device. Since a minimum of 2 V is needed to actuate, water electrolysis is a drawback that must be overcome, once its occurrence inhibits the properly working of ionomeric polymer/metal composite (IPMC). With individually four distinct counterions incorporated H+, Li+, Na+ and BMIM+— an organic cation from ionic liquid 1-butyl-3-methyl-imidazolium chloride — Nafion/Pt-based IPMCs had their electrochemical properties investigated through cyclic voltammetry, multi-step chronoamperometry and electrical impedance, as well as mass change during exhaustive actuation cycles and their chemical affinity with hydrophilic sites were analysed by quantum mechanics simulation. From the collected data, it was possible to infer that the physical-chemical aspects of counterions play a key role in device-measured property, and BMIM+ prevented early water electrolysis, providing electrochemical stability to the device. Real-time mass-monitored actuation shows that water loss occurs excessively on smaller radius counterions and although the fast reabsorption kinetics, molecular diffusion to the innermost regions of the membrane is slow and does not lead to a recovery of device’s performance. Electrochemical impedance spectroscopy data corroborate the fact that mass transport is strongly affected by both the limited ionomeric channel width and the interaction between the species and the Nafion side chains.