Structure and resistance of concentration polar layer on cation exchange membrane-solution interface
(School of Chemistry and Chemical Engineering, Central South University,
Changsha 410083, China)
Changsha 410083, China)
Abstract: Membrane/solution interface consists of a neutral concentration polar layer(CPL) and a charge layer(CL) under external electrical field, and the neutral CPL can be neglected under high frequency AC electrical field. The relationship of CL thickness e with electrolyte concentration C and fixed ion exchange sites density σ in membrane surface layer can be expressed as e=σ/C. According to this model, the thickness of the CL on Nafion1135 membrane/solution interface(ec) was calculated under different membrane surface charge quantity Q and variable electrolyte concentration C. The membrane/solution interface CL thickness(em) is obviously related with the membrane properties, and decreases dramatically in a higher electrolyte concentration. em values are 76.3 nm and 110.3 nm respectively for Nafion1135 and PE01 ion exchange membrane in 0.05 mol/L H2SO4 solution, and em values for both membrane tend to 2 nm in 2 mol/L H2SO4 solution. For Nafion1135 membrane, the comparison of ec and em gives the result that CL thickness em obtained by resistance measurement fits well with the calculated CPL thickness ec while proton in CL transferred to membrane surface is 14.56×10−10 mol, which corresponds to the fixed exchange group number in a surface layer with a thickness τ=2 nm for Nafion1135 membrane.
Key words: ion exchange membrane; AC electrical field; AC impedance; membrane/solution interface; structure; resistance