Comments on the Problem of Metal Ions Crossing through the Cations Membrane Electrolyte in Electrolysers and Redox Flow Cells

Redox flow Cells and hydrogen produced electrolysers are important green source of energy of the future. These cells can be activated to store energy (redox flow cells) and produce CO free hydrogen (electrolysers) by solar, wind or sea wave’s energy. In this Redox flow batteries and electrolysers a cations exchange membrane (e.g. Nafion from Du Point a cations exchange membrane selective to cations only) is used to separate the anodic regions from the cathodic regions. It is observed experimentally that during operations of these cells metal ions crosses from one region of the cell to the other which greatly lowers the efficiency of the process (irreversible capacity loss). This problem was partially solved in the case of Fe (II)/Fe(III) and Cr(II)/Cr(III) redox flow battery by utilizing the condition of electro-neutrality[1,2].The idea of mixed electrolyte introduced by Hagedorn [1] from NASA is based is using mixed cations in both anodic and cathodic regions so that during operation of the Fe (II)/Fe(III) and Cr(II)/Cr(III) redox flow cells the cations get reduced or oxidized instead of crossing the membrane and the condition of electro-neutrality is satisfied. The purpose of this publication is to generalize the application of the condition of electro-neutrality to other redox flow batteries and electrolysers and to give simple mathematical modeling of this effect [3].

 [1] Introduction of mixed reactants, elevated temperatures and additive electrocatalysts NASA TM-83401.

[2] Gahn, R. ; Hagedorn, N.; and Ling, J. S.: Single Cell Performance Studies on the Fe/Cr Redox Energy Storage System Using Mixed Reactant Solutions at Elevated Temperature. Proceedings of the Eighteenth Intersociety Energy Conversion Engineering Conference, AIChE, 1983, pp. 1647-1652.

[3] Rakib et al., Journal of Applied Electrochemistry 29: 1439, 1999.