Correlation between the proton conductivity and diffusion coefficient of sulfonic acid functionalized chitosan and Nafion composites via impedance spectroscopy measurements

Electrochemical Impedance Spectroscopy (EIS) was employed to estimate the global transverse proton diffusion coefficient, DH+, in sulfonic acid functionalized sustainable chitosan (CS-SO3H)/Nafion composite films. In contrast to conventional conductivity measurements, EIS measurements were performed at room temperature with a film/liquid interface. In this configuration, the measure of the bulk proton transport is correlated to the DH+ of the membranes which is close to 1.1 × 10−6 cm2 s−1 and 0.33 × 10−6 cm2 s−1 with and without CS-SO3H, respectively. These DH+ values permitted the proton conductivity (σH+) ratio (∼3.9) between the Nafion/CS-SO3H composite and pristine Nafion films to be estimated by using the Nernst-Einstein relationship. This ratio presents a good agreement with that obtained for the σH+ of bulk membranes (∼3.2) measured at 30 °C and 90% RH. The agreement between the σH+ ratios validates our methodology for DH+ estimation by EIS and suggests that the more than three times enhanced σH+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\sigma}_H^{+} $$\end{document} is governed by the ∼3 times higher DH+ in the presence of CS-SO3H.