Alkalinity Enhancement during Reject Brine Electrolysis: Role of Electrocatalyst Placement on the Outer Surfaces of Porous Flow-Through Electrodes

Membraneless electrolyzers offer a promising approach to convert environmentally harmful reject brine waste streams generated from desalination into valuable products, such as mineral carbonates, by using electrochemical reactions to split the brine into acidic and alkaline streams. However, membraneless electrolyzers suffer from a trade-off between current density and current utilization that stems from undesired back-reactions that arise from the crossover of redox species between the anode and cathode. This study employed a combination of in situ high-speed video, colorimetric pH imaging, modeling, and electroanalytical methods to evaluate how the performance of a porous flow-through cathode is affected by the operating current density, electrolyte flow rate, and choice of catalyst placement on a porous support. It is found that the placement of the active catalyst on the outer surface of the electrode─facing away from the anode─increases the cathode current utilization by 51%, on average, relative to when the catalyst is deposited on the inner surface of the electrode. This finding is explained by the ability of the porous electrode support to serve as a barrier to suppress crossover for the outward-facing catalyst configuration. In addition, the outward-facing catalyst configuration leads to a more stable operation while incurring minor increases (90–170 mV) in overpotentials. For both catalyst configurations, this study also shows that the Damköhler number (Da) is a practical descriptor for predicting operating conditions that maximize the concentration of OH– in the cathode effluent stream.