Direct evidence for fixed ionic groups in the hydrogel of an electrolyte diode

Current voltage characteristics of polyvinyl alcohol (PVA)-glutardialdehyde hydrogel cylinders were measured in dilute KCl solutions. The gel was the same type which had been applied in previous electrolyte diode experiments. Below 10(-2) molar KCl concentration the measured characteristics were nonlinear and at higher voltages the current was unstable. These results were explained by concentration polarization phenomena due to fixed negative charges present in the hydrogel and by electroconvection in the anodic boundary layer of the gel cylinder where a strong electric field can appear. To confirm these hypotheses further experiments were performed where spatially resolved concentration polarization data could be obtained inside the gel and in its close neighbourhood. The proposed model was also supported with semi-quantitative simulations which were able to reproduce the measured nonlinear characteristics and the observed concentration polarization phenomena. The fixed negative charges are most probably ionized carboxylic acid groups contaminating the PVA and the PVA-based hydrogel.