Amphoteric Metal Ion-Coordinated Chitosan Membranes for Efficient Hydroxide Ion Transport

Hydroxide exchange membranes (HEMs) have attracted extensive attention in energy-related fields, such as fuel cells and water electrolysis, primarily due to their suitability for alkaline environments. However, the practical application of membranes is hindered significantly by their limited conductivity. In this study, a series of amphoteric metal ion-coordinated chitosan (CTS-AM) membranes with enhanced hydroxide ion conductivity are reported. The CTS-AM membranes are prepared using a simple soaking-drying method, exhibiting excellent mechanical strength and thermal stability due to the strong coordination bonds between amphoteric metal ions and chitosan chains. To the best of our knowledge, the Zn²⁺ coordinated chitosan membrane achieved the highest-ever reported hydroxide ion conductivity of 82.0 ± 5.4 mS cm^-1 at 25 °C and 100% RH, with the value increasing to 301.0 ± 6.7 mS cm^-1 at operating temperature (80 °C) and 100% RH. Through combined structural analysis and theoretical calculations, we propose that the formation of nanochannels and the lowered barrier for electron transfer are responsible for the high hydroxide ion conductivities of CTS-AM membranes. This study presents a viable approach to the design and fabrication of high-performance HEMs for energy storage devices and other applications.