Electrode system for large-scale reverse electrodialysis: water electrolysis, bubble resistance, and inorganic scaling

It is generally accepted that the effect of electrode resistance is not predominant in determining the performance of reverse electrodialysis (RED), because the contribution of electrode resistance to total internal resistance decreases as the number of cell pairs increases. However, this is not true under the condition in which gas is continuously produced by water electrolysis owing to the large stack voltage in pilot-scale applications. We verified that the bubble resistance of the electrode spacer in a conventional endplate causes the electric power of a RED system with 1000 cells to decrease by more than 20% under the specific condition in which the outermost feed solution (OFS) at both electrodes and the electrode solution (ES) are river water. This configuration, called OFS(river)/ES(river), is the best for minimizing inorganic scaling and toxic gas evolution. Another problem associated with the conventional endplate is fluid congestion owing to very narrow spaces, which causes sudden pH changes and deteriorates further with inorganic scaling. To address these issues, we removed the electrode spacer from the electrode system and utilized an open-type endplate with interconnected open spaces. This endplate maintained high electric power without the bubble resistance and suppressed the abrupt changes in the pH around the electrodes and the shielding membranes. We believe that our approach will be useful in the search for an optimum electrode design for RED systems on the industrial scale.