Direct measurement of boundary layer thickness on ion-exchange membrane surfaces during reverse electrodialysis

A two-compartment electrolytic cell was constructed to simulate the membrane-solution system in reverse electrodialysis. Based on this setup, a potential analysis method was proposed to measure the thickness of the diffusion boundary layer on the membrane surface. By adjusting the position of the moving electrode in the system and measuring the potential difference between the fixed electrode and the moving electrode, the potential distribution on the membrane surface was obtained. The thickness of the diffusion boundary layer was then determined through analysis of the potential distribution. Additionally, the effects of various operating conditions on the diffusion boundary layer thickness were evaluated. The results indicated a significant decrease in the thickness of the diffusion boundary layer with increasing flow rate and temperature, thereby suppressing concentration polarization. Moreover, the thickness of the diffusion boundary layer exhibited an increase with the concentration of high salt solution and a decrease with the concentration of low salt solution. Furthermore, this study found that differences in membrane parameters had a non-negligible effect on the boundary layer thickness based on the use of four commercially available membranes.