Design Rules for Porous Flow-through Electrodes Used in Membrane-Free Electrolyzers for Treatment of Desalination Reject Brine

Abstract

The world’s growing demand for fresh water is creating increasing stress on existing fresh water sources and environmental hazards related to the generation of high salinity reject brine streams that are produced during seawater desalination. Electrochemical separations technologies offer attractive sources to treat brines to reduce brine salinity and/or harvest valuable chemicals or materials from dissolved species present in the brine. In this work, our team has explored the use of membrane-free electrochemical electrolyzers for treatment and separation processes through their ability to create large pH differentials between anode and cathode effluent streams. Because these devices do not contain membranes, they are attractive for operating in the harsh brine environment, which contains many impurities that can result in membrane scaling or fouling. In the first part of this talk, I will describe the basic operating principles of these electrolyzers, which rely on the use of flow-induced separation of anode and cathode effluent streams to create large pH gradients. During operation, it is generally desirable to maximize pH differentials by choosing flow conditions that limit cross-over of product species, thus ensuring high current utilization, while avoiding unnecessary dilution of H+ and OH- within the product streams. This trade-off between cross-over and dilution was studied for porous flow-through electrodes, where we systematically explore the combined influences of volumetric flow rates, catalyst placement, and operating current densities on the pH differential generated by electrolyzers fed with pH neutral brine solutions. Aided by the use of in situ high speed videography and colorimetric imaging of pH differentials, these experiments show that the Damköhler number is an excellent descriptor that can be used to identify the optimal operating conditions that maximize pH differential. Further, we show that the in-plane distribution of active catalyst loading on the porous electrode support has a large impact on the maximum pH differential that can be achieved. Finally, this presentation will also describe the tolerance of electrode durability to the presence of dissolved alkaline earth metal ions such as Mg2+, which can deposit on cathodes and result in scaling if not properly managed.