INVESTIGATION OF ELECTRODE MATERIAL-REDOX COUPLE FOR REVERSE ELECTRODYALISIS PROCESSES. PART I: IRON REDOX COUPLES.

The performances of electrodialysis (ED) and reverse electrodialysis (RED) processes depend on several factors, including the nature of the electrode material and of the redox couple adopted to make possible the conversion between electric power and chemical potential. In this paper, the possible utilization of iron-based redox couples (FeCl3/FeCl2, hexacyanoferrate(III)/hexacyanoferrate(II) and Fe(III)-EDTA/Fe(II)-EDTA) on graphite and DSA electrodes for RED processes was studied by a detailed experimental investigation. The hexacyanoferrate(III)/hexacyanoferrate(II) system was stable for lonf time (more than 12 days) in the absence of light and oxygen at high redox couple concentrations and low current densities both at compact graphite and DSA electrodes. Perfluorinated Nafion cationic membranes were found to be impermeable to the components of the redox couple. Fe(II)-EDTA exhibited a limited electrochemical stability in long term electrolyses at all adopted operative conditions, which discourages the use of the Fe(III)-EDTA/Fe(II)-EDTA for RED applications. The FeCl3/FeCl2 system was, on the other hand, stable for long times (more than 12 days) at acidic pH at compact graphite electrodes. Selemion anionic membranes allowed to confine the redox couple in the electrode compartments with very slow passage of protons to the side compartment (“dilute or concentrated compartment”).