Membraneless Electrolyzers for the Conversion of Brine into High-Value Products

Abstract

Desalination offers an approach to address the shortage of fresh water, however, the main concern is the generation of reject brine or concentrated saltwater that contains organic compounds, metals, and contaminants. Membraneless electrolyzers present a viable solution to convert the reject brine into H2 and O2, while simultaneously splitting it into acidic (HCl) and base (NaOH). They transport ions through a highly conductive liquid electrolyte instead of a membrane. The lack of a separator allows for operation in a wide range of pH environments and inhibits degradation and fouling issues found in conventional electrolyzers. In this study, a membraneless electrolyzer was 3D printed and a Pt|porous electrode and planar Pt|Ti-foil were used as the cathode and anode, respectively. The electrodes are a key component since that is where the reactions of interest (hydrogen and oxygen evolution reactions) occur. Hence, the distribution of Pt onto a carbon foam substrate in a uniform and asymmetric pattern was studied. Kinetic overpotentials and mass transfer limitations decrease when Pt nanoparticles are in a uniform pattern since it increases active sites resulting in faster kinetics. However, asymmetric catalyst placement controls where gaseous bubbles are generated and therefore decreases product crossover. The device’s ability to generate acid and base at different applied current densities and volumetric flow rates was investigated whereby at high current densities and low volumetric flow rates, the pH differential of the effluent streams broadens. However, at this condition, product crossover increases. Future work involves using in situ high-speed videography to determine the critical Damköhler number to unravel the achievable maximum pH while minimizing product cross-over.