SELECTION OF REDOX SYSTEMS FOR REVERSE ELECTRODIALYSIS PROCESSES

Abstract

In reverse electrodialysis (RED) processes, electrical energy is directly extracted from chemical potential gradients arising from salinity differences, especially from sea and river water. In RED there are at least four complementary elements: (1) electrodes, where electron transfer reactions occur to allow the transformation of the charge carrier from ion to electron; (2) ion selective exchange membranes, which allow the selective transport of ions; (3) solvents, which make a continuum for ion transport; (4) electrolytes, i.e. the current carriers between cathode and anode. Studies on RED processes were mainly focused on membranes but also on several other aspects including electrolyte compositions and concentration, modeling and fluidodynamics. Less attention has been given to the selection of the electrodic material-redox couple system (for the purpose of this work defined electrode system) with very few exceptions , the most relevant being a very recent paper of Veerman and co-authors that carried out a very detailed comparative assessment of the suitability for RED of selected electrode systems described in literature [1]. On the other hand, the behavior of these systems was rarely experimentally investigated under operative conditions of interest for RED or electrodialysis (ED) applications. Electrode systems can be grouped in two categories: with or without opposite electrode reactions [1]. In the first case, when recirculation of electrode rinse solution is adopted, no net modification of the chemical composition occurs and the electrodic thermodynamic voltage is null. The opposite electrode reactions can involve reactive electrodes such as in the systems Cu-CuSO4 , Ag-AgCl, Zn-ZnCl2 or homogeneous redox couples with inert electrodes [1]. This work was devoted to the study of the 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. To evaluate the advantages and disadvantages of these processes, numerous experiments were carried out in undivided and divided cells and in stack for the generation of energy. The Hexacyanoferrate(III)/Hexacyanoferrate(II) system was stable for long times 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, that discourages the use of the Fe(III)EDTA/Fe(II)EDTA for RED applications. The system FeCl3/FeCl2 was, on the other hand, stable for long times 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.