Understanding ammonia's role in mitigating concentration polarization in anion-exchange membrane electrodialysis.

Abstract

In processes such as electrodialysis, the applied electrical potential is constrained by concentration polarization at the membrane/solution interface. This polarization, which intensifies at higher current densities, impedes ion transport efficiency and may lead to problems such as salt precipitation, membrane degradation, and increased energy consumption. Therefore, understanding concentration polarization is essential for enhancing membrane performance, improving efficiency, and reducing operational costs. This study investigates the impact of ammonia buffer (NH₄ ⁺/NH₃) on sulfate ion transport through anion-exchange membranes with a particular focus on limiting current density and concentration polarization under constant current conditions. The findings demonstrate that ammonia effectively eliminates concentration polarization and enhances chemical reactions at the membrane interface. Notably, the plateau region was absent in the current-voltage curves, as was the transition time in the chronopotentiograms. Furthermore, the Warburg impedance arc in the Nyquist plot of the electrochemical impedance spectra was absent in both limiting and overlimiting current regions and the increasing dominance of the Gerischer arc was registered. At an ammonia concentration of 0.1 M, the influence of concentration polarization on mass transport was effectively mitigated, enabling sulfate counterions to pass through the membrane without encountering concentration polarization. The addition of ammonia catalytically accelerated the proton-transfer reactions, which accelerated the water dissociation reaction at earlier polarization stages, preventing the formation of diffusion boundary layers and facilitating the transport of sulfate counterions through the AMX anion-exchange membrane. As a result, the polarization plateau disappeared and the overlimiting current region shifted closer to the ohmic region, all without affecting the limiting current density (j _lim ).